Dugan was born in Wheaton, Illinois, the second of four sons of Marion, a housewife, and Charles Dugan, an insurance salesman. He graduated from Goodman School of Drama at the Art Institute of Chicago (now at DePaul University) and started his acting career in 1969 in New York City. He moved to Hollywood in 1972 and appeared in his first TV show, “The Sixth Sense”. Later, he played in the 1973 TV movie The Girl Most Likely to…. Other early film appearances include Night Call Nurses (1972), Night Moves (1975), Smile (1975), Harry and Walter Go to New York (1976) and Norman… Is That You? (1976). In 1979, he was cast as the time-displaced hero in Unidentified Flying Oddball, Disney’s very loose adaptation of A Connecticut Yankee in King Arthur’s Court. In 1981, he appeared in Joe Dante’s adaptation of The Howling.

Dugan’s first marriage was to actress Joyce Van Patten in 1973. The couple divorced in 1987, and he later wed Sharon O’Connor, to whom he is still married.

Dugan made guest appearances on several TV series during the 1970s including The Mod Squad, The Waltons, Cannon, Columbo, The Rockford Files, and Alice. He also appeared in the mini-series Rich Man, Poor Man, as well as two episodes of M*A*S*H and in 1978 starred in the short-lived series Richie Brockelman, Private Eye, a spin-off from The Rockford Files.

In June 2009, Dugan’s son, Kelly Dugan, was drafted with the 75th overall selection by the Philadelphia Phillies in the Major League Baseball Draft. A graduate of Notre Dame High School of Sherman Oaks, California, he has played for four of the club’s minor league teams, including the Gulf Coast League Phillies, Reading Fightin’ Phils, Williamsport Crosscutters, and Lakewood BlueClaws. As of 2020, Kelly was playing for the independent Lancaster Barnstormers.