Leonard was called to the bar in 1951. He entered the chambers of John Buzzard and specialised in criminal cases. He generally though not invariably appeared for the prosecution. In 1964, he became the Old Bailey second junior prosecuting counsel, and took silk in 1969. His "Telegraph" obituary describes him as a "shrewd prosecutor and deadly cross-examiner", adding that his style was "fair and understated, precise rather than flamboyant." As a barrister, he was involved in the prosecutions of Emil Savundra for fraud, of the Kray twins for the murders of Frank Mitchell and Jack “The Hat” McVitie, of Graham Young for poisoning several colleagues with thallium, and of George Ince and others for the so-called "Barn Murder" of Muriel Patience. He also repeatedly defended Patrick Armstrong, one of the Guildford Four, whose conviction for the Woolwich and Guildford pub bombings was eventually overturned.