He was born Ian David McGeachy in New Malden, Surrey, England. His parents divorced when he was five; Martyn spent his childhood alternating between England and Scotland.
His professional musical career began when he was 17; a blend of blues and folk resulting in a unique style that made him a key figure in the London folk scene during the mid-1960s. He signed to Chris Blackwell's Island Records in 1967 and released his first album, London Conversation, the following year.
This first album was soon followed by The Tumbler which was moving towards jazz. By 1970 Martyn had developed a wholly original and idiosyncratic sound: acoustic guitar run through a fuzzbox, phase-shifter, and Echoplex. This sound was first apparent on 1970s Stormbringer, which also had Martyn's wife, Beverley Kutner, as his collaborator. She also appeared on Road to Ruin.
In 1973, Martyn was to release one of the defining British albums of the 1970s, Solid Air, the title song a tribute to the singer-songwriter, Nick Drake, who was later to commit suicide in 1974. On this album, as with the one which preceded it, Bless the Weather, Martyn collaborated with jazz bass player, Danny Thompson.
His steady output has gained him considerable recognition as a performer and a songwriter.