In the 1960s, Leppard played an instrumental role in the rebirth of interest in baroque music; in particular, he was one of the first major conductors to perform baroque opera.
His interest in early music prompted him to prepare several realisations of scores from the period. While musicologists considered his editions controversial, his performances were important for introducing early operatic masterpieces to the general public. His production of Monteverdi's L'incoronazione di Poppea was presented at the Glyndebourne Festival under his direction in 1962. In the following years he subsequently prepared more operas by Monteverdi, as well as operas by Francesco Cavalli.
In 1963 he wrote the soundtrack for Peter Brook's film adaptation of William Golding's novel "The Lord of the Flies".
In November 1969, he made his USA debut conducting the Westminster Choir and New York Philharmonic, at which occasion he also appeared as soloist in the Haydn's D major Harpsichord Concerto. In 1973 he became principal conductor of the BBC Northern Symphony Orchestra in Manchester, a position he retained until 1980.
Leppard has also conducted Britten's Billy Budd at the Metropolitan and San Francisco Operas, Alceste and Alcina at the New York City Opera. He has also conducted at the Royal Opera House, Covent Garden and in Paris, Hamburg, Santa Fe, Stockholm, and Geneva.
At Glyndebourne, he conducted the world premiere of Nicholas Maw's Rising of the Moon. From 1987-2001, Leppard was the music director of the Indianapolis Symphony Orchestra, where he collaborated with concertmaster Hidetaro Suzuki for 14 seasons. For 2004-2006 he served as Music Advisor to The Louisville Orchestra. Leppard also serves as Artist-in-Residence at the University of Indianapolis.
Raymond Leppard has been honoured by The Queen with the CBE. In 1973 the Republic of Italy conferred upon him the title of Commendatore della Republica Italiana for services to Italian music.