This series allowed him to appear on several television series, and was even nominated for an Emmy Award. He also made several appearances in films such as Sweet Home Alabama, Made of Honor, Valentine's Day, Flypaper, Freedom Writers, Transformers: Dark of the Moon and Can not Buy Me Love.
Dempsey was born in Lewiston, Maine, and grew up in Buckfield, Maine. His mother, Amanda Caisson, worked as a secretary in a school, and his father, William A. Dempsey, was an insurance salesman. He attended schools Buckfield High School and St. Dominic Regional High School, which he left before graduation.
At twelve years old he was diagnosed with dyslexia. In an interview with Barbara Walters in 2008, stated that he believes that dyslexia helped 'shape the man he is today "and who has promoted" to keep fighting. "He added that "it has never given up."