Williamson began acting professionally as a child. His first TV appearances include "Starsky and Hutch", "Father Murphy", "Hill Street Blues", "Miami Vice", "China Beach", and "Midnight Caller". Perhaps his best-known television role was as program director Donovan Aderhold in the syndicated series "The New WKRP in Cincinnati". Williamson also starred in PBS's TV series "The Righteous Apples". The show focuses on the activities of The Righteous Apples, five Boston-area high school musicians, who in a troubled world, seek to help people in distress. Williamson was the lead singer of the group in the show in which he was just a teenager at that time.