Callahan played a doomed soldier/journalist in the M*A*S*H episode Sometimes You Hear the Bullet. 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.