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 the 1950s and 1960s, Leonard, a close friend of cartoonist Gerard Hoffnung's, participated both as conductor and composer, in the celebrated Hoffnung concerts. In 1959, at Leonard Bernstein's request, Leonard conducted the London premiere of West Side Story. Leonard was Associate Conductor of the BBC Northern Orchestra and Assistant Conductor to Sir John Barbirolli at the Hallé Orchestra. In 1964, Leonard conducted the world premiere of Gian Carlo Menotti's chamber opera Martin's Lie, at the Bath International Music Festival. From 1968 to 1973, Leonard was Music Director of the Edmonton Symphony Orchestra, where two of his orchestral compositions were premiered. In 1971, he conducted the Edmonton Symphony in a concert featuring the British rock bandProcol Harum, which was recorded and released commercially as , 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". In 1977, he arranged Mussorgsky’s Pictures at an Exhibition for piano and orchestra. In later years, he was Professor of Conducting at the Guildhall School of Music and Drama and Morley College, and regularly conducted the Royal Academy of Music Chamber Orchestra.
Original orchestral compositions
In addition to the aforementioned adaptation of Machaut's Grande Messe de Notre Dame and Group Questions for Orchestra, Leonard's original orchestral compositions include: Four Pieces, Four Contrasts, Break, Processional, A Short Overture, A Swinging Tune, and the symphonic poem Mezoon.
Books
Leonard wrote The Horn of Mortal Danger, a children's adventure tale set in a secret civilisation underneath London. It was a 'completely new departure' from anything he had done before. He also authored the irreverent yet insightful book 1812 and All That: A Concise History of Music from 30,000BC to the Millennium.
