Pratt started his career in the chorus of the D'Oyly Carte Opera Company in 1945, moving up to small roles and then understudying Martyn Green, the principal comedian.
[3] In September 1951, exactly six years after joining the company, Pratt became the principal comedian following Green's departure,[4] and he served in that capacity for the next eight seasons, playing Sir Joseph Porter in Pinafore, the Major-General in The Pirates of Penzance, Bunthorne in Patience, the Lord Chancellor in Iolanthe, Ko-Ko in The Mikado, Robin in Ruddigore, Jack Point in The Yeomen of the Guard, the Duke of Plaza-Toro in The Gondoliers and King Gama in Princess Ida.
[3][5] In 1953, his first London season as principal comedian, The Times reviewed him in the role of the Lord Chancellor: "Mr. Peter Pratt... had a good delivery, a quiet manner, and a nimble pair of legs that contrasted suddenly and superbly with his dry demeanour.
[6] In reviewing a 1956 Ruddigore, The Times wrote, "Mr. Pratt showed true operatic talent in the [twin roles] changing the colour of his tone and the expression of his face with decisive skill.
During his tenure with the D'Oyly Carte Opera Company, Pratt recorded the roles of Major Murgatroyd (Patience, 1951), John Wellington Wells (The Sorcerer, 1953), King Gama (Princess Ida, 1955), Ko-Ko (The Mikado, 1957), and Major-General Stanley (Pirates, 1957).
[12] In 1966, BBC Radio presented a complete cycle of the thirteen extant Gilbert and Sullivan operas, with dialogue, with Pratt starring in ten of them and working behind the scenes as co-producer.
[15] In 1976, Pratt appeared in the serial The Deadly Assassin from the BBC's long-running science fiction television series Doctor Who, becoming the second actor to play the Master.
[19] He also appeared as a soloist in the concert video recording "Gilbert & Sullivan Present their Greatest Hits," made at Royal Albert Hall in 1982.