When the Big Sound split at the end of 1967 during a tour of Norway, some members, including Wallace, moved to London to back other artists such as Sandie Shaw, David Garrick, Marv Johnson and Lou Christie.

In May 1972, at the end of a U.S. tour, he and fellow Crimson members Mel Collins and Boz Burrell left the band and went to work for Alexis Korner's Snape.

Wallace's studio and live credits also include El Rayo-X with David Lindley, Bonnie Raitt, Joe Walsh, Bob Dylan, Johnny Hallyday, Keith Emerson, Roy Orbison, Jackson Browne, the Traveling Wilburys, Eric Clapton, Jon Anderson, Alvin Lee, Crosby, Stills and Nash, the Quireboys, Larry Coryell, Stevie Nicks, Lindsey Buckingham, Steve Marriott, Al Kooper, Tim Buckley, Lonnie Mack, Procol Harum (1993 tour), and Warren Zevon.

Among his later studio recordings there were sessions with Kim Richey, Tim Krekel, Rick Vito, Dean Dillon, Rosie Flores, Jessi Alexander, producer Gary Nicholson, Steve Ripley, Jan Pulsford, Tim Hinkley, Charlie Taylor, Rodney Crowell and the legendary songwriter Dan Penn.

He also performed live with T. Graham Brown, the Nashville Chamber Orchestra, Rick Vito, Jessi Alexander, and Billy Burnette, the latter in a quartet that included bassist Dave Roe (Johnny Cash) and Kenny Vaughan (Lucinda Williams).