Around 1726 Leclair found his way to Turin to study with violinist-composer G.B. Somis, as well as to compose ballet interludes (now lost) for opera productions at the Teatro Regio Ducale. A second opus of violin sonatas was published in 1728, and during that same year Leclair made his debut as a violinist at the Concert Spirituel. Performances of his own music in London, Kassel (where Leclair engaged in a musical "duel" with famed Italian violinist Pietro Locatelli) and Paris earned Leclair a reputation as one of the leading figures of the new French school of violinist-composers. Formal recognition came in 1733 with an appointment to the musical court of Louis XV, to whom Leclair dedicated his third opus of violin sonatas as a display of gratitude.
Leclair divided his time between a number of court appointments for the remaining decades of his life, including positions at The Hague, at the court of Orange in the Netherlands, and in the service of the Duke of Gramont (a former pupil of the composer). The official investigation of Leclair's suspicious death in 1767 incriminated both his nephew and his second wife (the couple had lived in separate households since 1758), but neither was ever formally charged with his murder.
Leclair took the Italian sonata da chiesa and the sonata da camera and infused them with a stylistic elegance derived largely from the ballet music of Jean-Baptiste Lully. Fittingly, he composed almost exclusively for the violin (he did compose one opera in 1746, but the work never entered the Parisian repertory). His significance as a teacher of the violin, however, is perhaps greater than his place in the annals of composition: with a string of pupils including notable French violinists L'abbé le fils, Jean-Josephe Rudolphe, and Pierre Gaviniès, Leclair can truly be called the father of the modern French violin school.