This British violinist is most famous for his involvement with the Melos Ensemble, an extended chamber group that was formed in 1950 by the viola player Cecil Aronowitz. The group, with a dozen available members, formed in order to play larger chamber works, including the Schubert and Mendelssohn octets, Beethoven's Septet, and Ravel's Introduction and Allegro. The ensemble had no trouble establishing an international reputation in an area where, due to both budgetary and scheduling problems, long-running groups are rare. The expanded size of this group included a string quintet, wind quintet, harp, and piano. The Melos Ensemble was intensely involved in the premiere of Britten's War Requiem in 1962 and also became known for many premieres of new chamber works. These performances included Birtwistle's Tragoedia and Maxwell Davies' Seven In Nomine, both in 1965. Other members of the ensemble include clarinetist Gervase de Peyer, flautist Richard Adeney, cellist Terence Weil, and first violinist Emanuel Hurwitz, who left the group in the early 1970s. McMahon appeared with the Melos group regularly at British and international festivals, including performances in Venice, Warsaw, and throughout Holland. The group toured the United States for the first time in 1966, and repeatedly after that. The BBC network featured the group on many broadcasts, and Melos Ensemble left behind a rich legacy of some 50 recordings.
McMahon also performed with many orchestras. Most notably, he played with the London Philharmonia Orchestra from 1952, under conductor Arturo Toscanini. He was part of that orchestra's tour of the United States in 1955, under conductor Herbert von Karajan.