In 1435 he was sent by Cardinal Albergati, Eugenius IV's legate at the council, on a secret mission to Scotland, the object of which is variously related even by himself. He visited England as well as Scotland, underwent many perils and vicissitudes in both countries, and has left a valuable account of each. The journey to Scotland proved so tempestuous that Piccolomini swore that he would walk barefoot to the nearest shrine of Our Lady from their landing port. This proved to be Dunbar, and the nearest shrine 10 miles distant at Whitekirk. The journey through the ice and snow left Aeneas afflicted with pain in his legs for the rest of his life.
Upon his return to Basel, Aeneas sided actively with the council in its conflict with the Pope, and, although still a layman, obtained a leading share in the direction of its affairs. He supported the creation of the antipope Felix V (1439–1449), Amadeus, Duke of Savoy. Aeneas then withdrew to the Emperor Frederick III's (1440–1493) court at Vienna. He was crowned imperial poet laureate, and obtained the patronage of the Emperor's chancellor, Kaspar Schlick. Some identify the love adventure at Siena Aeneas related in his romance, The Tale of the Two Lovers with an escapade of the Chancellor.
Aeneas' character had hitherto been that of an easy man of the world, with no pretence to strictness in morals or consistency in politics. He now began to be more regular in the former respect, and in the latter adopted a decided line by making his peace with Rome. Being sent on a mission to Rome in 1445, with the ostensible object of inducing Eugenius to convoke a new council, he was absolved from ecclesiastical censures, and returned to Germany under an engagement to assist the Pope. This he did most effectually by the diplomatic dexterity with which he smoothed away differences between the papal court of Rome and the German imperial electors; and he had a leading part in the compromise by which, in 1447, the dying Eugenius IV accepted the reconciliation tendered by the German princes, and the council and the antipope were left without support. He had already taken orders, and one of the first acts of Eugenius IV's successor Pope Nicholas V (1447–1455) was to make him bishop of Trieste. He later served as bishop of Siena.
In 1450 Aeneas was sent as ambassador by the Emperor Frederick III to negotiate his marriage with the princess Eleonore of Portugal, which object he successfully achieved; in 1451 he undertook a mission to Bohemia, and concluded a satisfactory arrangement with the Hussite leader George of Podebrady; in 1452 he accompanied Frederick III to Rome, where the Emperor wedded Leonora and was crowned King of the Romans. In August 1455 Aeneas again arrived in Rome on an embassy to proffer the obedience of Germany to the new Pope Calixtus III (1455–1458). He brought strong recommendations from the Emperor and King Ladislaus of Hungary for his nomination to the cardinalate, but delays arose from the Pope's resolution to promote his own nephews first, and he did not attain the object of his ambition until December in the following year. He achieved temporarily the bishopric of Warmia (Ermeland).
On his return from the congress, Pius II spent a considerable time in his native district of Siena, where he was joined by his erstwhile host in Mantua Ludovico Gonzaga; Pius has described his delight and the charm of a country life in very pleasing language. He was recalled to Rome by the disturbances occasioned by Tiburzio di Maso, who was ultimately seized and executed. In the struggle for the Kingdom of Naples between the supporters of the House of Aragon and the House of Anjou, the Papal States were at this time troubled by rebellious barons and marauding condottieri, whom he gradually, though momentarily, abated. The Neapolitan War was also terminated by the success of the Pope's ally the Aragonese Ferdinand. In particular, the pope engaged for most of his reign in what looked a personal war against Sigismondo Pandolfo Malatesta, lord of Rimini, with the result of the almost complete submission of that condottiero. Pius II tried also mediations in the Thirteen Years' War between Poland and the Teutonic Knights, but, when he failed to achieve success, cast an anathema over Polish and Prussians both. Pius II was also engaged in a series of disputes with the Bohemian King George of Podebrady and the Sigismund of Austria (who was excommunicated for having arrested Nicholas of Cusa, bishop of Brixen).
In July 1461, Pius II canonized Saint Catherine of Siena, and in October of the same year he gained at first what appeared to be a most brilliant success by inducing the new King of France, Louis XI (1461–1483), to abolish the Pragmatic Sanction of Bourges, by which the Pope's authority in France had been grievously impaired. But Louis XI had expected that Pius II would in return espouse the French cause in Naples, and when he found himself disappointed he virtually re-established the Pragmatic Sanction by royal ordinances.
The crusade for which the Congress of Mantua had been convoked made no progress. The Pope did his best: he addressed an eloquent letter to the sultan urging him to become a Christian, a letter that probably never was sent. Not surprisingly, if it was delivered, this invitation was not successful. A public ceremony was staged to receive the relics of the head of Saint Andrew when it was brought from the East to Rome. Pius II succeeded in reconciling the Emperor and the King of Hungary, and derived great encouragement as well as pecuniary advantage from the discovery of mines of alum in the papal territory. But France was estranged; the Duke of Burgundy broke his positive promises; Milan was engrossed with the attempt to seize Genoa; Florence cynically advised the Pope to let the Turks and the Venetians wear each other out. Pius II was unaware he was nearing his end, and his malady probably prompted the feverish impatience with which on June 18, 1464, he assumed the cross and departed for Ancona to conduct the crusade in person. It seemed certain that the issue of such an enterprise could only be ridiculous or disastrous. Pius II's good genius again stepped in, and rendered it pathetic.
Pius II had sincere, loving nature, frank and naïve even in his aberrations and defects. The leading trait of Pius II's character was his extreme impressionableness. Chameleon-like, he took colour from surrounding circumstances, and could always depend on being what these circumstances required him to be. As, therefore, his prospects widened and his responsibilities deepened, his character widened and deepened too; and he who had entered upon life a shifty character quit it a model chief shepherd. While he vied with any man in industry, prudence, wisdom, and courage, he excelled most men in simplicity of tastes, constancy of attachments, kindly playfulness, magnanimity, and mercy. As chief of the church he was able and sagacious, and showed that he comprehended the conditions on which its monopoly of spiritual power could for a season be maintained; his views were far-seeing and liberal; and he was but slightly swayed by personal ends.
Pius II was a versatile and voluminous author, one of the best and most industrious of his period. His most important and longest work is his autobiography Commentaries in 13 books, first published in 1584 by Cardinal Francesco Bandini Piccolomini, a distant relative. Piccolomini altered it to some extent, removing words, phrases and whole passages that were unflattering to his relative. Piccolomini published it under the name of scribe Gobellinus, who was then misattributed as the author, a natural mistake because Pius II chose to write Commentaries from the third-person perspective. Pius II was also the author of numerous erotic poems and an obscene comedy titled Chrysis (such ethics were not unusual for his period).
His Epistles, which were collected by himself, are also an important source of historical information. The most valuable of his minor historical writings are his histories of Bohemia and of the Emperor Frederick III, the latter partly autobiographical. He sketched biographical treatises on Europe and Asia, and in early and middle life produced numerous tracts on the political and theological controversies of his day, as well as on ethical subjects. Pius II was greatly admired as a poet by his contemporaries, but his reputation in belles lettres rests principally upon his The Tale of the Two Lovers, which continues to be read to this day, partly from its truth to nature, and partly from the singularity of an erotic novel being written by a Pope. He also composed some comedies, one of which alone is extant. All these works are in Latin. Pius was not an eminent scholar: his Latin is frequently incorrect, and he knew little Greek; but his writings have high literary qualities.
Pope Pius II inaugurated an unusual urban project, perhaps the first city planning exercise in modern Europe. He refurbished his home town which is now by his name called Pienza (province of Siena, Tuscany). A cathedral and palaces were built in the best style of the day to decorate the city. They survive to this day.