Withrow was born in Highland Heights, Kentucky, the son of Nancy and Thomas Withrow. He has one older brother, Jeff, and one younger brother, Roger. Withrow attended Campbell County High School in Alexandria, Kentucky. Glenn was very active in high school sports, playing on the football team and was runner up in the state wrestling tournament in spite of the fact that he had the flu during the competition.
Withrow began acting in the mid 1970s after moving to Los Angeles, and had roles in films such as The Outsiders as Tim Shepard, Rumble Fish and Peggy Sue Got Married. He has had roles in five productions directed by Francis Ford Coppola.
He is the CEO of In House Media, Inc., a film production company, who produced the feature film "The Mooring" under their subsidiary partnership, In House Media Film Partners. He has also taught acting classes with his wife Hallie Todd.
He directed the full-length feature film The Mooring, which was released on DVD, digital download and Video on Demand on February 19, 2013 by Lionsgate. Withrow also produced and co-wrote the film with his wife and daughter.
He will be directing his second film, The Last Champion, in November 2016.
Withrow has been married to actress Hallie Todd since 1991. The couple have a daughter, Ivy, born in 1993.