Ainsley made his official solo debut in 1987 when he sang in Stravinsky's "Mass" under Simon Rattle. After further engagements with Gothic Voices and other groups he made his professional opera debut in 1988 as Armindo in Scarlatti's "Gli equivoci nel sembiante" at Innsbruck's Early Music Festival. This was followed by his debut with English National Opera the following year as Eurimaco in "Il ritorno d'Ulisse in patria", and his United States debut in 1990 in a series of concerts in Boston and New York. He has subsequently performed all around the world in operas and concerts. In 1992 he made his debut with the Berlin Philharmonic under Haitink and created the tenor part in the world premiere of John Tavener's "We shall see Him as He is". The following year he made his debut in the Vienna Musikverein, singing the Evangelist in Bach's "St Matthew Passion" and tenor solos in the "St John Passion" under Peter Schreier. Other notable concert appearances include performances with the London Philharmonic under Roger Norrington, the London Symphony under Sir Colin Davis, Mstislav Rostropovich and André Previn, Les Musiciens du Louvre under Marc Minkowski, the Cleveland Orchestra under Franz Welser-Möst, the New York Philharmonic under Kurt Masur, the Boston Symphony under Seiji Ozawa, the San Francisco Symphony under Jeffrey Tate, the Vienna Philharmonic under Trevor Pinnock, the Academy of St Martin in the Fields under Neville Marriner, and both the Orchestra of the Maggio Musicale Fiorentino and the Orchestre de Paris under Carlo Maria Giulini among many others.