McPartlin was born in Chicago, Illinois, the son of Steve and Lois McPartlin. He was raised in Glen Ellyn, Illinois (a suburb of Chicago), and attended Glenbard South High School. McPartlin graduated with a degree in speech communication from the University of Illinois at Urbana–Champaign. He was a member of the Illinois Fighting Illini football team as a walk-on tight end from 1993-95. McPartlin's older brother, Chris, was also a member of the Illinois football team, earning a varsity letter as a linebacker in 1994. After six months in Australia and New Zealand, McPartlin moved to Southern California to pursue acting as a career.