John Mitchinson became one of the most important British tenors in the 1950s and into the 1980s, particularly in heroic roles and oratorios.
He studied at the Royal Manchester College of Music. While still a student there he joined the BBC Northern Singers in 1953, becoming one of its founding members. Upon graduation, he sang with the Chelsea Opera Group. This semi-professional group, which was founded in 1950 by the young conductor Colin Davis to give concert performances of Mozart operas, was an important means of entrée for talented young singers, including Mitchinson, whose debut performance in operatic music was in one of Chelsea's concert presentations of Mozart's Don Giovanni.
Mitchinson did not debut on stage until 1959, when he sang the part of Jupiter in Handel's Semele at the Handel Opera Society. He had a notable career in the 1960s, including participation in Leonard Bernstein's pioneering recording of Mahler's Symphony No. 8, "The Symphony of a Thousand." He also sang on Jascha Horenstein's stereo recording of Liszt's Faust Symphony in 1958 and Colin Davis' recording of Berlioz's Beatrice and Benedict.
Other items that were important to his repertory included the roles of Wagner's Tristan, Florestan in Beethoven's Fidelio, Britten's Peter Grimes, and Aegisthus in Strauss' Elektra, and Stravinsky's Oedipus. He was also noted for his interpretation of Mahler's Das Lied von der Erde. He was on the roster of the English National Opera from 1972 to 1978 and the Welsh National Opera from 1978 to 1982. While a member of the WNO he participated in the historic performance under the baton of Charles Mackerras (recorded on Supraphon Records) of Czech composer Bohuslav Martinu's operatic masterwork The Greek Passion.