Treadwell was born in Long Island, New York, one of five children of Val and Carol Dexter. He attended Connetquot High School, where he achieved B average grades and was the swimming team's star diver. He was also very fond of animals, and kept a squirrel named Willie as a pet. In an interview with his parents in the film Grizzly Man, they 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 his account, he became an alcoholic after he lost the role of Woody Boyd to Woody Harrelson in the sitcom Cheers.[3]