Drafted in the 7th round of the 1997 NFL draft, Detmer spent the next 10 years in Philadelphia. He spent most of his career as a backup to quarterback Donovan McNabb and as the place-kick holder for David Akers. His action as a starter was limited to five games in the 1998 season and one game in the 2002 season, in which McNabb was out with a broken ankle. Detmer played well in that game, going 18 of 26 for 227 yards and three total touchdowns (two passing and one rushing). He dislocated his elbow in the fourth quarter of that game and was replaced by third-string quarterback A. J. Feeley. Detmer healed enough to play in the final games of 2002, but head coach Andy Reid decided to stay with Feeley, who was having success. He also saw sporadic playing time in the 2004 and 2005 seasons.