Harrington was selected by the Detroit Lions with the third pick overall in the 2002 NFL Draft. Harrington took over for incumbent Mike McMahon late in the Lions' Week 1 loss against the Miami Dolphins and became the Lions' starting quarterback shortly thereafter, finishing that year with a 50.1 completion percentage, a ratio of 12 touchdowns to 16 interceptions, and a 59.9 quarterback rating; the Lions finished the season with a 3–13 record. He was named the 2002 recipient of the Detroit Lions/Detroit Sports Broadcasters Association Rookie of the Year Award.