The 1984Detroit Lions season was their 55th in the National Football League. The team failed to improve upon their previous season's output of 9–7, winning only four games. The team missed the playoffs for the first time in three seasons. In a Lions season littered with numerous key injuries, the most painful was star running back Billy Sims suffering a career-ending knee injury in a game against the Minnesota Vikings during the season. In just five seasons in the NFL since joining Detroit in 1980, Sims had set the Lions career rushing mark at 5,106. 
1984 would prove the last time until 1996 that the Lions played the San Diego Chargers. The reason for this is that before the admission of the Texans in 2002, NFL scheduling formulas for games outside a team’s division were much more influenced by table position during the previous season.