Thomas remained in London, hooking up with Bitter Sweet and Bodast in the late 1960s and recording with Quiver, The Sutherland Brothers, Moonrider and Al Stewart in the early 1970s.

Thomas attended Grangefield Grammar School in Stockton on Tees, and after leaving, worked as a trainee commercial-artist at the local Evening Gazette newspaper. He was playing bass in local bands at night, around his home town, and eventually joined the Roadrunners (with Paul Rodgers & Micky Moody). After Rodgers switched from playing bass to being their up-front singer, the band changed their name to The Wildflowers, and in late 1966 decided to quit their respective day-jobs and move to London. After achieving little success, the band eventually split.

His inventive and highly melodic bass work with Costello brought Thomas his greatest fame. Between 1977 and 1987, Elvis Costello & The Attractions released nine record albums, including This Year's Model (1978), Punch the Clock (1983), and Blood & Chocolate (1986), and toured extensively.

After Costello's initial split with the Attractions in 1987, Thomas recorded with such other artists as Billy Bragg, John Wesley Harding, and Suzanne Vega. In 1990 he released his first book The Big Wheel, a memoir in which the key characters are recognizable without ever being identified by name. Costello, for instance, is called "the singer." Apparently annoyed by his depiction in the book, Costello responded with the song "How To Be Dumb" on his album Mighty Like a Rose (1991). Costello has described his relationship with Thomas during this period as "pretty non-existent".

Thomas has also been a nonfiction Writer for several decades, and is the author of 1994's Bruce Lee: Fighting Spirit, a biography of the renowned martial Artist and movie star, and 1991's The Body of Time (and the Energies of Being), ISBN 0-14-019301-4, a short book about metaphysics. Rough Notes, released in 2015, is both a memoir of his time with The Attractions, and a documentary history of the British music scene of the 1970s.

Despite this estrangement, Costello was persuaded by co-producer Mitchell Froom to invite Thomas to play on the album Brutal Youth (1994). Thomas was featured on 5 tracks of the album. The reunited Elvis Costello & the Attractions followed the album with a tour and another album All This Useless Beauty (1996). Toward the end of a second tour, Costello announced that he would be splitting with the group as soon as the tour was over. Elvis Costello & the Attractions played their final concert on 15 September 1996, in Nagoya, Japan.

In 2003 Thomas was inducted into the Rock and Roll Hall of Fame as a member of Elvis Costello and the Attractions. Thomas appeared onstage with Costello to accept the honour, but they did not perform together. Thomas's book On the Road... Again, the sequel to The Big Wheel, was published that same year.

Costello has said in interviews with a variety of publications, including the Rocky Mountain News, that the old animosity between himself and Thomas returned. Costello claimed that Thomas deliberately sabotaged some songs onstage, which was the breaking point. The other two Attractions, Steve Nieve and Pete Thomas (no relation to Bruce), have continued to tour and record with Costello as The Imposters, with Davey Faragher, formerly of Cracker, on bass. Bruce Thomas has said that he simply lost interest in playing with Costello. In an interview Thomas stated "no reformation is ever going to come about under my initiative – or under any other circumstances I can foresee."