Benjamin attended Westminster School and then studied with Olivier Messiaen at the Paris Conservatoire during the second half of the 1970s. Messiaen was reported to have described Benjamin as his favourite pupil.
He then read music at King's College, Cambridge, studying under Alexander Goehr, and emerged in his early twenties as a mature and confident voice. His orchestral piece Ringed by the Flat Horizon (written for the Cambridge University Musical Society and premiered in Cambridge under the baton of Mark Elder on 5 March 1980) was performed at The Proms that August, while he was still a student, making him the youngest composer ever to have had music performed at the Proms.
Since the 1980s he has fulfilled a number of large commissions, including Sudden Time (for orchestra), Three Inventions (for chamber orchestra) and Antara (for ensemble and electronics, realised at IRCAM and the first composition ever published using the Sibelius notation program).
In 1993, Benjamin curated the first 'Meltdown' music festival in London. In the 2002-2003 concert season, the London Symphony Orchestra gave a season-long festival of concerts which he curated, called "By George!.
Benjamin taught composition at the Royal College of Music, London, before he succeeded Sir Harrison Birtwistle as Henry Purcell Professor of Composition at King's College London in January 2001. He lives in London and regularly conducts the London Sinfonietta. Benjamin has been a teacher and mentor to such younger composers as Luke Bedford.
Benjamin's oeuvre been described as exhibiting "consummate craftsmanship" coloured by "a love of rich and unusually coloured sonorities".