Guy Longnon was born in the 6th arrondissement of Paris on July 16, 1924. He studied cello in a class at the Conservatoire de Paris and became a trumpeter, and he frequented Boris Vian and the musicians of Saint-Germain-des-Prés. By the beginning of the 1950s he was a working trumpeter in the Paris traditional and mainstream scene, playing with Raymond Fol, Don Byas, Sidney Bechet, Mezz Mezzrow, Claude Luter, Albert Nicholas, Gérard Pochonet and the drummer Moustache, and in the orchestras of Michel Attenoux and André Réwéliotty. English Wikipedia counts about sixty jazz recording sessions between 1950 and 1958.

His earliest recorded credit on Discogs is trumpet in Raymond Fol's orchestra on the 78 Ivory Black (Swing, 1950). Discogs lists him on trumpet for the Sidney Bechet and Claude Luter orchestra at the Salle Pleyel concerts of January 31 and March 12, 1952, which Blue Note issued in 1953 as the two volumes of Jazz Festival Concert, Paris 1952. He then appears on records by the clarinettist Albert Nicholas (the Columbia 10-inch Albert Nicholas Quartet and Sextet, 1956, and the March 1954 session reissued in Classic Jazz at Saint-Germain-des-Prés), by Mezz Mezzrow (the Vogue LP Swingin' With Mezz, 1962, and the 1955 session at the Schola Cantorum), by Don Byas (three tracks recorded in early 1951), and by the Moustache Jazz Seven. Discogs also credits him on trumpet on a run of tracks by Hubert Fol's Be-Bop Minstrels, collected on a 2018 compilation, and he wrote the music for two short films by Paul Paviot, Terreur en Oklahoma (1951) and Chicago Digest (1952).

The work for which he is remembered in France came after the Paris years. In 1963, on the initiative of the pianist Pierre Barbizet, director of the Marseille conservatoire, he created a jazz class at the Conservatoire national à rayonnement régional de Marseille. French Wikipedia describes it as the first jazz class in a French conservatoire, and the critic Gérard Ponthieu called him, in his 2014 obituary, the first to bring jazz into one. Among the students who came out of the class are Bruno Angelini, André Jaume, Pierre Christophe and Jean-Marc Padovani. The trombonist Philippe Renault succeeded him in 1992.

No record under his own name is documented. In 2013 the group D6, made up of his former students and directed by Yves Laplane, recorded the tribute album Kind of Guy, for which Longnon wrote all the arrangements. He passed away in Marseille in February 2014, at the age of 89. He was the uncle of the trumpeter Jean-Loup Longnon.

A Paris-born musician, he built his career in Paris from 1950 and in Marseille from the early 1960s.

Longnon came up in the New Orleans-revival and mainstream jazz of postwar Paris, the repertoire of Bechet, Luter, Mezzrow and Nicholas, and Discogs also credits him on trumpet with Hubert Fol's Be-Bop Minstrels. The trumpeter's own tastes ran toward bebop: the saxophonist André Jaume remembered a lecture he gave in Marseille around 1960 in which he stated his preference for bebop, breaking with Hugues Panassié, the head of the Hot Club de France, and said that by then he had grown tired of being seen as a singer's accompanist when playing with Bechet. As a teacher he was remembered by Jaume as modest and open to every kind of music, from classical to jazz, citing Duke Ellington, Charlie Parker and Clifford Brown, and as playing piano or double bass in his classes rather than trumpet. His reputation rests more on the generation of Marseille musicians he trained than on a leader discography.