[citation needed] On television, Campbell became known to viewers when he co-starred with John Ritter on the short-lived Three's Company spin-off Three's a Crowd (1984–1985) in which he played the comedic role of E.Z.
He also co-starred for five seasons with William Conrad and Joe Penny as Assistant District Attorney Derek Mitchell on the CBS crime drama Jake and the Fatman (1987–1992).
His most current Broadway role was playing Sam Carmichael in the hit musical Mamma Mia!
He has appeared in several regional theatrical productions of Race, Death and the Maiden, 12 Angry Men, Hay Fever, Of Thee I Sing, Oleanna, Beauty and the Beast, Johnny Guitar, Bells Are Ringing, On Shiloh Hill, Boogie Woogie Rumble of a Dream Deferred and Breakfast at Tiffany's, and productions of I Love My Wife and The Nerd.
On April 24, 2008, Campbell and Kennedy opened the Lauren Kennedy and Alan Campbell Theater, which establishes a home for a burgeoning theater program at Barton College, a small liberal arts school in Wilson, North Carolina.