Tim was born and raised in St. Louis, Missouri. He was somewhat shy and overweight as a boy, and got into theater because he enjoyed making people laugh. In high school he was part of a traveling theater show doing the lights, and when one of the actors became sick he went on instead. He loved it, and realized that he was more comfortable in front of a crowd than relating to people one on one. Dunigan went on to graduate from Webster University with a degree in drama. He worked his way through college by appearing in musicals and waiting on tables. He later played the part of Templeton Peck "The Faceman" in the pilot of The A Team but because he looked too young to be a Vietnam vet he was replaced by Dirk Benedict.