On his father's retirement in 1847, the family moved to Frankfurt, partly from motives of economy and partly for the boy's education. Here Jenkin and his father spent a pleasant time together, sketching old castles, and observing the customs of the peasantry. At thirteen, Jenkin had produced a romance of three hundred lines in heroic couplets, a novel, and innumerable poems, none of which are now extant. He learned German in Frankfurt and, on the family migrating to Paris the following year, he studied French and mathematics under a M. Deluc. While there, Jenkin witnessed the outbreak of the Revolution of 1848 and heard the first shot, describing the action in a letter written to an old schoolfellow 
The Jenkins left Paris, and went to Genoa, where they experienced another revolution, and Mrs. Jenkin, with her son and sister-in-law, had to seek the protection of a British vessel in the harbour, leaving their house stored with the property of their friends, and guarded by Captain Jenkin. At Genoa, Jenkin attended the University, being its first protestant student. Padre Bancalari, the professor of natural philosophy, lectured on electromagnetism, his physical laboratory being the best in Italy. Jenkin took the degree of M.A. with first-class honours, his special subject having been electromagnetism. The questions in the examinations were in Latin, and had to be answered in Italian. Fleeming also attended an art school in the city, and gained a silver medal for a drawing from one of Raphael's cartoons. His holidays were spent in sketching, and his evenings in learning to play the piano or, when permissible, at the theatre or opera-house. He had conceived a taste for acting.
At home he pursued his studies, and was for a time engaged with Dr. Bell in working out a geometrical method of arriving at the proportions of Ancient Greek architecture. His stay in Manchester, though in striking contrast to his life in Genoa, was agreeable. He liked his work, had the good spirits of youth, and made some pleasant friends, one of them the author, Elizabeth Gaskell. He was argumentative, and his mother tells of his having overcome a Consul at Genoa in a political discussion when he was only sixteen, simply from being well-informed on the subject, and honest. He is as true as steel, she writes, and for no one will he bend right or left... Do not fancy him a Bobadil; he is only a very true, candid boy. I am so glad he remains in all respects but information a great child.
On leaving Fairbairn's he was engaged for a time on a survey for the proposed Lukmanier Railway in Switzerland, and in 1856 he entered Penn's engineering works at Greenwich as a draughtsman, being occupied on the plans of a vessel designed for the Crimean War. He complained about the late hours, his rough comrades, and his humble lodgings, across a dirty green and through some half-built streets of two-storied houses.... Luckily, he adds, I am fond of my profession, or I could not stand this life. Jenkin had been his mother's pet until then, and felt the change from home more keenly for that reason. At night he read engineering and mathematics, or Thomas Carlyle and the poets, and cheered his drooping spirits with frequent trips to London to see his mother.
Another social pleasure was his visits to the house of Alfred Austin, a barrister, who became permanent secretary to Her Majesty's Office of Works and Public Buildings, and retired in 1868 with the title of CB. His wife, Eliza Barron, was the youngest daughter of a gentleman of Norwich who, when a child, had been patted on the head, in his father's shop, by Dr Samuel Johnson, while canvassing for Mr. Thrale. Jenkin had been introduced to the Austins by a letter from Mrs. Gaskell, and was charmed with the atmosphere of their choice home, where intellectual conversation was happily united with kind and courteous manners, without any pretence or affectation. Each of the Austins, says Stevenson, in his memoir of Jenkin, was full of high spirits; each practised something of the same repression; no sharp word was uttered in the house. The Austins were truly hospitable and cultured, not merely so in form and appearance. It was a rare privilege and preservative for a solitary young man in Jenkin's position to have the entry into such elevating society, and he appreciated his good fortune.
Annie Austin, their only child, had been highly educated, and knew Greek among other things. Though Jenkin loved and admired her parents, he did not at first care for Annie. Stevenson hints that she vanquished him by correcting a false quantity of his one day; he was the man to reflect over a correction, and admire the castigator. Jenkin was poor, but the liking of her parents for him gave him hope. He had entered the service of Messrs. Liddell and Gordon, who were engaged in the new work of submarine telegraphy, which satisfied his aspirations, and promised him a successful career. He therefore asked the Austins for leave to court their daughter. Mrs. Austin consented freely, and Mr. Austin only reserved the right to inquire into his character. Jenkin, overcome by their disinterestedness, exclaimed in one of his letters, Are these people the same as other people? Miss Austin seems to have resented his courtship of her parents first but the mother's favour, and his own spirited behaviour, saved him, and won her consent.
After leaving Penn's, Jenkin became a railroad engineer under Liddell and Gordon, and, in 1857, became engineer to R. S. Newall & Co. of Gateshead who shared the work of making the first Atlantic cable with Glass, Elliott & Co. of Greenwich. Jenkin was busy designing and fitting up machinery for cableships, and making electrical experiments. I am half crazy with work, he wrote to his fiancee; I like it though: it's like a good ball, the excitement carries you through. He wrote, My profession gives me all the excitement and interest I ever hope for.... I am at the works till ten, and sometimes till eleven. But I have a nice office to sit in, with a fire to myself, and bright brass scientific instruments all round me, and books to read, and experiments to make, and enjoy myself amazingly. I find the study of electricity so entertaining that I am apt to neglect my other work.... What shall I compare them to, he writes of some electrical experiments, a new song? or a Greek play?
Another attempt was made the following year, but with no better success. Brett then tried to lay a three-wire cable from the steamer Dutchman but owing to the deep water (in some places 1500 fathoms or 2700 m) when he came to a few miles from Galita, his destination on the Algerian coast, he had not enough cable to reach the land. He telegraphed to London for more cable to be made and sent out, while the ship remained there holding the end. After five days the cable parted, perhaps as a result of rubbing on the bottom.
It was to recover the lost cable of these expeditions that the Elba was got ready for sea. Jenkin had fitted her out the year before for laying the Cagliari to Malta and Corfu cables but on this occasion she was better equipped. She had a new machine for picking up the cable, and a sheave or pulley at the bows for it to run over, both designed by Jenkin, together with a variety of wooden buoys, ropes, and chains. Liddell, assisted by F. C. Webb and Fleeming Jenkin, was in charge of the expedition. Jenkin had nothing to do with the electrical work, his care being the deck machinery for raising the cable but it was a responsible job. He reported the expedition in letters to Miss Austin and in diary entries 
During the latter part of the work much of the cable was found to be looped and twisted into 'kinks' from having been so slackly laid and two immense tangled skeins were raised on board, one by means of the mast-head and fore-yard tackle. Photographs of this ravelled cable were exhibited as a curiosity in the windows of Newall & Co.'s shop in The Strand. By 5 July the whole of the six-wire cable had been recovered and a portion of the three-wire cable, the rest being abandoned as unfit for use, owing to its twisted condition. On the evening of the 2nd the first mate, while on the water unshackling a buoy, was struck in the back by a fluke of the ship's anchor as she drifted, and so severely injured that he lay for many weeks at Cagliari. Jenkin's knowledge of languages made him useful as an interpreter but, in mentioning this incident to Miss Austin, he writes, For no fortune would I be a doctor to witness these scenes continually. Pain is a terrible thing.
On 26 February, during a four days' leave, Jenkin married Miss Austin at Northiam, returning to his work the following Tuesday. He was strongly attached to his wife and his letters reveal a warmth of affection which a casual observer would never have suspected in him. In 1869 he wrote, People may write novels, and other people may write poems, but not a man or woman among them can say how happy a man can be who is desperately in love with his wife after ten years of marriage. Five weeks before his death he wrote to her, Your first letter from Bournemouth gives me heavenly pleasure - for which I thank Heaven and you, too, who are my heaven on earth.
In 1863 his first son was born and the family moved to a cottage at Claygate near Esher. Though ill and poor, he kept up his self-confidence. The country, he wrote to his wife, will give us, please God, health and strength. I will love and cherish you more than ever. You shall go where you wish, you shall receive whom you wish, and as for money, you shall have that too. I cannot be mistaken. I have now measured myself with many men. I do not feel weak. I do not feel that I shall fail. In many things I have succeeded, and I will in this.... And meanwhile, the time of waiting, which, please Heaven, shall not be so long, shall also not be so bitter. Well, well, I promise much, and do not know at this moment how you and the dear child are. If he is but better, courage, my girl, for I see light.
He took to gardening, without a natural liking for it, and soon became an ardent expert. He wrote reviews and lectured or amused himself in playing charades and reading poetry. James Clerk Maxwell was among his visitors. During October 1860, he superintended the repairs of the Bona-Spartivento cable, revisiting Chia and Cagliari, then full of Garibaldi's troops. The cable, which had been broken by the anchors of coral fishers, was grapnelled with difficulty. What rocks we did hook! writes Jenkin. No sooner was the grapnel down than the ship was anchored; and then came such a business: ship's engines going, deck engine thundering, belt slipping, tear of breaking ropes; actually breaking grapnels. It was always an hour or more before we could get the grapnels down again.
In 1865, on the birth of their second son, Mrs. Jenkin was very ill, and Jenkin, after running two miles for a doctor, knelt by her bedside during the night in a draught. He suffered from rheumatism and sciatica ever afterwards. It nearly disabled him while laying the Lowestoft to Norderney cable for Paul Reuter in 1866. This line was designed by Forde & Jenkin, manufactured by Messrs. W. T. Henley & Co., and laid by the Caroline and William Cory. Clara Volkman, a niece of Reuter, sent the first message, with C. F. Varley holding her hand.
The following June he was on board the Great Eastern while she laid the French Atlantic cable from Brest to Saint-Pierre. Among his shipmates were Sir William Thomson, Sir James Anderson, C. F. Varley, Latimer Clark and Willoughby Smith. Jenkin's sketches of Clark and Varley are remarkable. At Saint-Pierre they arrived in a fog which lifted to show their consort, the William Cory, straight ahead, and the Gulnare signalling a welcome. Jenkin observed that the whole island was electrified by the battery at the telegraph station.
In 1873 Thomson and Jenkin were engineers for the Western and Brazilian cable. It was manufactured by Hooper & Co., of Millwall and the wire was coated with india rubber, then a new insulator. The Hooper left Plymouth in June, and after touching at Madeira, where Thomson was up sounding with his special toy (the pianoforte wire) at half-past three in the morning, they reached Pernambuco by the beginning of August, and laid a cable to Pará.
During the next two years the Brazilian system was connected to the West Indies and the Río de la Plata but Jenkin was not present on the expeditions. While engaged in this work, the ill-fated La Plata, carrying cable from the Siemens AG company to Montevideo, sank in a cyclone off Ushant with the loss of nearly all her crew. The Mackay-Bennett Atlantic cables were also laid under their charge.
In mechanical engineering his graphical methods of calculating strains in bridges, and determining the efficiency of mechanism, were valuable, and won him the Keith Gold Medal from the Royal Society of Edinburgh. He also founded the Sanitary Protection Association, for the supervision of houses with regard to health. In his spare time Jenkin wrote papers on a wide variety of subjects. He attacked Darwin's theory of development, and showed its inadequacy, especially in demanding more time than the physicist could grant for the age of the habitable world. Darwin confessed that some of his arguments were convincing; and Munro, the scholar, complimented him for his paper on Lucretius and the Atomic Theory.' In 1878 he constructed a phonograph from the newspaper reports of this new invention, and lectured on it in Edinburgh, then employed it to study the nature of vowel and consonantal sounds. An interesting paper on Rhythm in English Verse,' was also published by him in the Saturday Review for 1883.
He could draw a portrait with astonishing rapidity, and had been known to stop a passer-by for a few minutes and sketch her on the spot. His artistic side also shows itself in a paper on 'Artist and Critic,' in which he defines the difference between the mechanical and fine arts. 'In mechanical arts,' he says, 'the craftsman uses his skill to produce something useful, but (except in the rare case when he is at liberty to choose what he shall produce) his sole merit lies in skill. In the fine arts the student uses skill to produce something beautiful. He is free to choose what that something shall be, and the layman claims that he may and must judge the artist chiefly by the value in beauty of the thing done. Artistic skill contributes to beauty, or it would not be skill; but beauty is the result of many elements, and the nobler the art the lower is the rank which skill takes among them.'
Jenkin was a clear and graphic writer. He read selectively, preferring the story of David, the Odyssey, the Arcadia, the saga of Burnt Njal, and the Grand Cyrus. Aeschylus, Sophocles, Shakespeare, Ariosto, Boccaccio, Sir Walter Scott, Dumas, Charles Dickens, William Makepeace Thackeray, and George Eliot, were some of his favourite authors. He was a rapid, fluent talker. Some of his sayings were shrewd and sharp; but he was sometimes aggressive. 'People admire what is pretty in an ugly thing,' he used to say 'not the ugly thing.' A lady once said to him she would never be happy again. 'What does that signify?' cried Jenkin ; 'we are not here to be happy, but to be good.' On a friend remarking that Salvini's acting in Othello made him want to pray, Jenkin answered, 'That is prayer.'
Though admired and liked by his intimates, Jenkin was never popular with associates. His manner was hard, rasping, and unsympathetic. 'Whatever virtues he possessed,' says Mr. Stevenson, 'he could never count on being civil.' He showed so much courtesy to his wife, however, that a Styrian peasant who observed it spread a report in the village that Mrs. Jenkin, a great lady, had married beneath her. At the Saville Club, in London, he was known as the 'man who dines here and goes up to Scotland.' Jenkin was conscious of this churlishness, and latterly improved. 'All my life,' he wrote,'I have talked a good deal, with the almost unfailing result of making people sick of the sound of my tongue. It appeared to me that I had various things to say, and I had no malevolent feelings; but, nevertheless, the result was that expressed above. Well, lately some change has happened. If I talk to a person one day they must have me the next. Faces light up when they see me. "Ah! I say, come here." " Come and dine with me." It's the most preposterous thing I ever experienced. It is curiously pleasant.'
Jenkin was a good father, joining in his children's play as well as directing their studies. The boys used to wait outside his office for him at the close of business hours; and a story is told of little Frewen, the second son, entering in to him one day, while he was at work, and holding out a toy crane he was making, with the request, 'Papa you might finiss windin' this for me, I'm so very busy to-day.' He was fond of animals too, and his dog Plate regularly accompanied him to the University. But, as he used to say, 'It's a cold home where a dog is the only representative of a child.'
In the Highlands, Jenkin learned to love the Highland character and ways of life. He shot, rode and swam well, and taught his boys athletic exercises, boating, salmon fishing, and so on. He learned to dance a Highland reel, and began the study of Gaelic; but it proved too difficult even for Jenkin. Once he took his family to Alt Aussee, in the Steiermark (Styria), where he hunted chamois, won a prize for shooting at the Schützenfest, learned the local dialect, sketched the neighbourhood, and danced the steirischen Ländler with the peasants.
His parents and parents-in-law had come to live in Edinburgh, but they all died within ten months of each other. Jenkin had showed great devotion to them in their illnesses, and was worn out with grief and watching. His telpherage, too, had given him considerable anxiety; and his mother's illness, which affected her mind, had caused him fear. He was planning a holiday to Italy with his wife in order to recuperate, and had a minor operation on his foot, which resulted in blood poisoning. There seemed to be no danger, and his wife was reading aloud to him as he lay in bed, when his mind began to wander. He probably never regained his senses before he died.
At one period of his life Jenkin was a Freethinker, holding all dogmas as 'mere blind struggles to express the inexpressible.' Nevertheless, as time went on he returned to Christianity. 'The longer I live,' he wrote, 'the more convinced I become of a direct care by God--which is reasonably impossible--but there it is.' In his last year he took Communion.