He started playing the trumpet at the age of twelve, and within a year had joined the National Youth Jazz Orchestra.

After lessons from Clark Terry in 1975, Barker went on in the 1980s to play with John Dankworth, Gil Evans (with whose orchestra he toured and recorded in 1983), Lena Horne, and Bobby Watson.

[1] As a sideman he has played with Ornette Coleman, Carla Bley, Georgie Fame, James Carter, Mike Westbrook, Frank Sinatra, Colin Towns, Natalie Merchant, ABC, The The, Haircut One Hundred, Erasure, Chris Botti, Wham!, Kajagoogoo, The Housemartins, Matt Bianco, Alphaville, The Style Council, Swing Out Sister, The Moody Blues, Sting, Bucks Fizz, Mike Oldfield, Cleo Laine, Acoustic Alchemy, XTC, and Stan Tracey.

More recently, he has toured with the Guy Barker Jazz Orchestra – a 15-piece big band featuring Rosario Giuliani (alto saxophone; Italy) and Per Johansson (tenor saxophone; Sweden) – performing DZF, a reworking of Mozart's Magic Flute with Michael Brandon narrating the story as a Raymond Chandler-style novel, retold by Robert Ryan.

[5] Barker was appointed Member of the Order of the British Empire (MBE) in the 2015 New Year Honours for services to jazz.