Along with other composers mainly in the Holy Roman Empire and France, he was responsible for certain operatic reforms including reducing ornateness of style and the primacy of star singers somewhat.

[1] His first opera, the comedy L'errore amoroso, was presented, with great success, under the protection of the Marquis del Vasto, Giovanni Battista d'Avalos, in the winter of 1737 at the Teatro Nuovo of Naples.

His first serious opera Ricimero rè de' Goti, presented in the Roman Teatro Argentina in January 1740, brought him to the attention and then the protection of the Duke of York, Henry Benedict.

The main result of his stay in Bologna and his association with Martini was to present to the Accademia Filarmonica of that city, as application for admission, his first known sacred composition, a five-voice fugue a cappella on the final words of the small doxology, "Sicut erat".

During the early 1740s he wrote an increasing amount of religious music, mainly oratorios, and his first liturgical piece still extant, a very simple "Lætatus sum" in F major dated 1743, is part of the Santini collection in Münster.

In the years immediately after this, he wrote operas for Venice, Turin, Bologna, Ferrara and Padua, and two popular oratorios, Isacco figura del Redentore and Betulia liberata.

Some time around 1745, Hasse recommended Jommelli for a position as the Director of Music at the Ospedale degli Incurabili in Venice, one of that city's colleges for female musicians.

But in 1745, he did start writing religious works for the women's choir of the church of the Incurabili, San Salvatore, as part of his obligations as chapel master along with teaching the more advanced students of the institution.

Jommelli wrote cantatas, oratorios and other sacred works, but by far the most important part of his output were his operas, particularly his opere serie of which he composed around sixty, several with libretti by Metastasio.