Married to soprano Faustina Bordoni and a friend of librettist Pietro Metastasio, whose libretti he frequently set, Hasse was a pivotal figure in the development of opera seria and 18th-century music.

[4] In October Hasse left Dresden to direct premieres of his next operas at Turin and Rome, and he also wrote music for the Venetian theatres at this time.

Soon after Hasse visited Venice and Munich, returning to Dresden in June 1747 to stage his opera La spartana generosa, performed to celebrate multiple royal weddings at this time.

The marriage of princess Maria Josepha of Saxony to the French Dauphin gave Hasse the opportunity to journey to Paris in the summer of 1750, where his Didone abbandonata was performed.

On 28 March 1750 Hasse presented his last oratorio La conversione di Sant' Agostino in the chapel of the royal palace in Dresden.

The libretto, penned by the Dresden Electress Maria Antonia Walpurgis (1724–1780), concerns the conversion of a sinner to sainthood and was modeled after and edited by Metastasio.

In 1756 the Seven Years' War compelled the court at Dresden to move to Warsaw, though Hasse himself lived mostly in Italy, travelling to Poland solely to supervise productions of his operas, if at all.

In the autumn of 1760 he moved to Vienna, where he stayed for the next two years, returning to Dresden in 1763 to find much of his home destroyed and the musical apparatus of the court opera wrecked.

[6] In 1764 Hasse travelled to Vienna, where the coronation of Joseph II was marked by a performance of his festa teatrale Egeria, again set to a libretto by Metastasio.

With the premiere of Piramo e Tisbe (September 1768) Hasse had intended to retire from opera but was compelled by Maria Theresa to compose a further work, Ruggiero (1771), again set to a Metastasian libretto.

The first, regarding all innovations as quackery, adhere to the ancient form of the musical drama, in which the poet and musician claim equal attention from an audience; the bard in the recitatives and narrative parts; and the composer in the airs, duos and choruses.

The second party depend more on theatrical effects, propriety of character, simplicity of diction, and of musical execution, than on, what they style flowery description, superfluous similes, sententious and cold morality, on one side, with tiresome symphonies, and long divisions, on the other.

[8]: 232–233 Finding his music under siege from an avant-garde surge in a new direction, Hasse left Vienna in 1773 and spent the final ten years of his life in Venice, teaching and composing sacred works.

In one of his letters, dated to March 1744, Metastasio made the following comments: ...never until now had I happened to see him [Hasse] in all his glory, but always detached from his many personal relationships in such a way that he was like an aria without instruments; but now I see him as a father, husband and friend, qualities which make an admirable union in him with those solid bases of ability and good behaviour, for which I will cherish him so many years...[This quote needs a citation] In the following years Hasse reset his earlier works based on Metastasio's texts, this time paying great attention to the poet's original intention, and during the 1760s, as Metastasio wrote new texts, Hasse was, as a general rule, the first composer to set them.

[10] Burney left the following note: This poet and musician are the two halves of what, like Plato's Androgyne, once constituted a whole; for as they are equally possessed of the same characteristic marks of true genius, taste, and judgement; so propriety, consistency, clearness, and precision, are alike the inseparable companions of both...[8]: 235–236 Despite Giovanni Battista Mancini's claim that Hasse was the padre della musica, and despite the composer's massive popularity as a figure at the very forefront of 18th-century serious Italian opera, after his death Hasse's reputation vastly declined and his music lay mostly unperformed (with the exception of some of his sacred works, which were revived now and again in Germany).