For several decades Lamont worked as a freelance musician (on flute and clarinet as well as saxophone), based around Archer Street in Soho and playing in the surrounding jazz clubs. He often performed with British bands accompanying American vocalists, including Fred Astaire, Tony Bennett, Bing Crosby, Sammy Davis Jr., Marlene Dietrich, Peggy Lee and (for 19 seasons) with Frank Sinatra. He also played with touring bandleaders such as Count Basie, Benny Goodman and Henry Mancini. Lamont led his own band for 11 years. He worked on recording and performing projects with Kenny Wheeler for many decades, including on Wheeler's 1974 album Song for Someone.
Lamont was a prolific composer of concert works, library music, television music and songs. The Young Person's Guide to the Contemporary Jazz Orchestra, composed for a BBC broadcast in 1979, provides a jazz orchestra alternative to Benjamin Britten's classical variations, also for orchestra and narrator. The Sherlock Holmes Suite was commissioned by the City of London to commemorate the first appearance of Sherlock Holmes in The Strand magazine, and featured his friend and collaborator Spike Milligan as narrator. It was recorded for the BBC in 1989.
Lamont began to record orchestral music for the KPM and Bruton music libraries in the 1970s. He became part of the library music recording group WASP, along with Steve Gray on piano, Brian Bennett on drums, Dave Richmond on bass and Clive Hicks on guitar. His music for television included the theme tunes for the BBC television children's animation series Mr Benn (using the name Don Warren) and Spot.
Blossom Dearie, Natalie Cole, Cleo Laine and Norma Winstone were among those who recorded his songs. Cleo Laine ended her Carnegie Hall show with his Not You Again. A CD of his songs by Nancy Marano was released in 1995. Frank Holder also recorded a set of the songs in the mid-1990s. In 2020 Tina May issued another song selection.
In May 2019 just over four weeks before he died, Lamont returned to the town of his birth, Greenock, to perform a homecoming gig with singers Esther Bennett and Daniela Clynes. His son Duncan Lamont Jr. stated, "There was real sense of everything having come full circle."
