[4] His musical career began at age 16, as an orchestral cellist with the London Symphony Orchestra, playing under such conductors as Sir Henry Wood, Richard Strauss, and Leopold Stokowski.
In 1964, Leonard conducted the world premiere of Gian Carlo Menotti's chamber opera Martin's Lie, at the Bath International Music Festival.
[5] In 1971, he conducted the Edmonton Symphony in a concert featuring the British rock band Procol Harum, which was recorded and released commercially as Procol Harum Live: In Concert with the Edmonton Symphony Orchestra, although due to an apparent distaste for rock music, from the start of the project Leonard disowned any association with the performance and went so far as to refuse to have his name listed in the credits.
The concert and resulting recording went on to become the most successful live pairing of symphony orchestra and rock music to date and a worldwide best-seller, and even spawned a hit single, "Conquistador".
][citation needed] In addition to the aforementioned adaptation of Machaut's Grande Messe de Notre Dame (1972) and Group Questions for Orchestra (1973), Leonard's original orchestral compositions include: Four Pieces, Four Contrasts, Break, Processional, A Short Overture, A Swinging Tune, and the symphonic poem Mezoon (written for the Sultan of Oman).