Harrington's career in Detroit was largely unsuccessful. Front office mismanagement, woeful offensive line protection, lack of talent at other skill positions, and an erratic philosophical change in the team's identity to a conservative West Coast Offense (WCO) oriented attack under Head Coach Steve Mariucci may have played a factor in Harrington not realizing his potential professionally, as well as his own play and lack of talent. Harrington's best season as a Lion came in 2004, when he threw for 19 touchdowns and 12 interceptions. The Lions started the season with a 4–2 record, but Harrington led the team to only two more wins the rest of the season. The Lions finished 6–10 and missed the playoffs for the fifth season in a row.