On 8 April 1891, Monash married Hannah Victoria Moss, and their only child, Bertha, was born in 1893. He worked as a civil engineer, introducing reinforced concrete to Australian engineering practice; he was the engineer of the Morrell Bridge/Anderson Street bridge over the Yarra River, Melbourne, which opened in 1899. He took a leading part in his profession and became president of the Victorian Institute of Engineers and a member of the Institution of Civil Engineers, London.
Monash joined the university company of the militia in 1884 and became a lieutenant in the North Melbourne battery militia unit in 1887. He was made captain in 1895, major in 1897 and in 1906 became a lieutenant-colonel in the intelligence corps. He was colonel commanding the 13th Infantry Brigade in 1912; on the outbreak of World War I he was appointed chief censor in Australia.
When war broke out in 1914, Monash became a full-time Army officer. Despite the anti-German hysteria of the time, there seems to have been no adverse comment on his German origins. When the Australian Imperial Force was formed, he was sent as commander of the 4th Infantry Brigade to Egypt.
In 1915 his brigade, as part of the New Zealand and Australian Division under Major General Godley, participated in the disastrous Gallipoli campaign against the Ottoman Army. The brigade initially defended the line between Pope's Hill and Courtney's Post, and the valley behind this line became known as "Monash Valley". There he made a name for himself with his independent decision-making and his organisational ability. He was promoted to brigadier-general in July.
During the August offensive, Monash's objective was the capture of Hill 971, the highest point on the Sari Bair range, but a failure to get his troops through poorly mapped mountainous terrain prior to the battle resulted in disaster for the last co-ordinated effort to defeat the Turkish forces on the Gallipoli Peninsula. This marked the lowest point of his military career.
He commanded the final significant assault of the Gallipoli fighting in the attack on Hill 60 on August 21, which was only partially successful. His war letters are full of accounts of the gallantry of the men he commanded. When orders came in December 1915 for the evacuation, he methodically supervised the exact course to be followed by members of his own command, and was in one of the last parties to leave.
Great as the disappointment had been over the failure at Gallipoli, there was some comfort in the fact that the evacuation had been so successful. Forty-five thousand men, with mules, guns, stores, provisions and transport valued at several million pounds, had been withdrawn with scarcely a casualty, and without exciting the slightest suspicion in the enemy. Hours afterwards the Turks opened a furious bombardment on the empty trenches.
After a rest period in Egypt, by June 1917 Monash was in north-west France. In July, with the rank of major-general, he was in charge of the new Australian 3rd Division. He trained the division in England with the minutest attention to detail, and led stage by stage to the nearest approach that could be improvised to the conditions of actual warfare. He was involved in many actions, including Messines, Broodseinde, and the First Battle of Passchendaele, with some successes, but with the usual heavy casualties. The British High Command was impressed by Monash's abilities and enthusiasm. In May 1918 he was promoted to lieutenant-general and made commander of the Australian Corps, at the time the largest corps on the Western Front.
Lieutenant-General Sir John Monash later described the recapture of the town of Villers-Bretonneux on 25 April 1918 after the Germans had overrun the 8th British Division under General William Heneker as the turning-point of the war. Sir Thomas William Glasgow`s 13th Brigade, and Harold Elliott's 15th Brigade, recaptured Villers-Bretonneux.
Monash, not being a professionally trained officer, was free of the antiquated doctrines of many First World War officers. He believed in the co-ordinated use of infantry, aircraft, artillery and tanks. He wrote:
Charles Bean, the official Australian war historian, noted that Monash was more effective the higher he rose within the Army, where he had greater capacity to use his skill for meticulous planning and organisation, and to innovate in the area of technology and tactics. Bean had been no great admirer of Monash in his early career, in part due to a general prejudice against Monash's Prussian-Jewish background, but more particularly because Monash did not fit Bean's concept of the quintessential Australian character that Bean was in the process of mythologising in his monumental work 'Australia in the War of 1914-1918'. (Both Bean and Monash, however, having seen the very worst excesses of British military doctrines and the waste of life on the Western Front, were determined that the role of the commander was to look after, and protect as far as possible, the troops under their command.) Bean, who had said of Monash "We do not want Australia represented by men mainly because of their ability, natural and inborn in Jews, to push themselves", conspired with Keith Murdoch to undermine Monash, and have him removed from the command of the Australian Corps. They misled Prime Minister Billy Hughes into believing that senior officers were opposed to Monash. Hughes arrived at the front before the Battle of Hamel prepared to replace Monash, but after consulting with senior officers, and after seeing the superb power of planning and execution displayed by Monash, he changed his mind.
At the Battle of Hamel on 4 July 1918 Monash applied his doctrine of "peaceful penetration", and led Australian Divisions, along with a small detachment of US troops, to win a decisive victory for the Allies. On 8 August 1918, the Battle of Amiens was launched. Australian troops under Monash and Canadian troops under Arthur Currie attacked the Germans. The battle was a strong, significant victory for the Allies, causing the Germans to recognise that for them the War was lost. The defeated German leader, General Ludendorff, described it in the following words: "August 8th was the black day of the German Army in the history of the war". On 12 August 1918 Monash was knighted as a Knight Commander of the Order of the Bath on the battlefield by King George V, the first time a British monarch had honoured a commander in such a way in 200 years. The Australians then achieved a series of victories against the Germans at Chignes, Mont St Quentin, Peronne and Hargicourt. Monash had 208,000 men under his command, including 50,000 inexperienced Americans. Monash planned the attack on the German defences in the Battle of the Hindenburg Line between 16 September and 5 October 1918. The Allies eventually breached the Hindenburg Line by the 5th of October, and the war was essentially over. On 5 October, Prinz Max von Baden, on behalf of the German Government, asked for an immediate armistice on land, water and in the air.
By the end of the war Monash had acquired an outstanding reputation for intellect, personal magnetism, management and ingenuity. He also won the respect and loyalty of his troops: his motto was "Feed your troops on victory". Field Marshal Bernard Montgomery later wrote: "I would name Sir John Monash as the best general on the western front in Europe".
Soon after the conclusion of hostilities Monash was placed in charge of a special department to carry out the repatriation of the Australian troops. He returned to Australia on 26 December 1919 to a tumultuous welcome.
Later, Monash worked in prominent civilian positions, the most notable being head of the State Electricity Commission of Victoria (SECV) from October 1920. He was also Vice-Chancellor of the University of Melbourne from 1923 until his death 8 years later. Monash was an active member of the Rotary Club of Melbourne, Australia's first Rotary Club, and served as its second President (1922-23).
He was called upon by the Victorian Government of Harry Lawson in 1923 to organise 'special constables' to restore order during the 1923 Victorian Police strike. He was one of the principal organisers of the annual observance of ANZAC Day, and oversaw the planning for Melbourne's monumental war memorial, the Shrine of Remembrance. Monash was honoured with numerous awards and decorations from universities and foreign governments. Monash was devastated in early 1929, when his eldest grandchild, John (who was 6 at the time), passed away after catching a rare influenza virus.
He died in 1931 in Melbourne, where the City of Monash, Monash Medical Centre (the location of his bust, which originally resided in former SECV town Yallourn), Monash Freeway and Monash University are named after him. His face is on Australia's highest value currency note ($100). Also named in his honour is Kfar Monash ("Monash village") in Israel. Monash's success in part reflected the tolerance of Australian society, but to a larger degree his success - in the harshest experience the young nation had suffered - shaped that tolerance and demonstrated to Australians that the Australian character was diverse, multi-ethnic, and a blend of the traditions of the 'Bush' and the 'city'.
In a final sign of humility, despite his achievements, honours and titles, he instructed that his tombstone simply bear the words "John Monash". He is buried in Brighton General Cemetery.