He is perhaps best known for his portrayal from 1987 to 1990 of Walter Powell on the syndicated sitcom Charles in Charge, starring Scott Baio.

While attending school in the Midwest, he discovered acting and on the advice of a teacher, he enrolled at the University of Washington in Seattle He studied drama and graduated in the late 1950s.

He also had recurring roles in three 1960s series, as Danny Adams in ABC's Wendy and Me, with a cast including George Burns, Connie Stevens, Ron Harper, and J. Pat O'Malley, as Lieutenant O'Connell on NBC's 13-episode war drama, Convoy, with John Gavin and John Larch, and as the governor's press secretary in The Governor and J.J., which starred Dan Dailey and Julie Sommars.

Callahan portrayed a small role in Barnaby Jones in an episode titled "Counterfall" (02/04/1975).

Six months later, on August 3, 2007, Callahan died at his home in Fallbrook, California at age 76.