Treadwell was born on Long Island, New York, one of five children of Val and Carol Dexter. He attended Connetquot High School, where he was the swimming team's star diver. He was very fond of animals and kept a squirrel named Willie as a pet. In an interview in "Grizzly Man" (2005), his parents say he was an ordinary young man until he went away to college. There, he claimed that he was a British orphan who was born in Australia. According to this account, his father said Timothy "spiraled down" and became an alcoholic after he lost the role of Woody Boyd to Woody Harrelson in the sitcom "Cheers".