Dempsey was born in Lewiston, Maine,[3] and grew up in the nearby towns of Turner and Buckfield. He has two sisters.[5] His mother, Amanda (née Casson),[6][7] was a school secretary, and his father, William, was an insurance salesman.[3][8][9]
He attended Buckfield High School and St. Dominic Regional High School.[5] He was an adept juggler, tying for second in a national juggling competition.[10]
Dempsey was diagnosed with dyslexia at age twelve. He told Barbara Walters on her 2008 Oscar special that he thinks dyslexia made him what he is today. "It's given me a perspective of—you have to keep working," Dempsey told Walters. "I have never given up."[11]