[5] Kosier attended Cactus High School in Glendale, Arizona,[6] where he was a letterman in football, basketball, baseball, and track and graduated in 1997.
[10] On April 19, 2005, the Detroit Lions signed him as a restricted free agent to a one-year contract, reuniting with his former 49ers head coach Steve Mariucci.
[11][12] Although the move was made to replace Larry Allen at left guard,[13] it didn't get much acknowledgment because Kosier was a relatively unknown player.
Kosier played that year with a plantar fascia injury, before suffering a torn medial collateral ligament in his left knee in the last game of the regular season.
[15] Throughout his Cowboys years, he always remained an important presence in the locker room, often getting recognition for the versatility, chemistry, and stability he provided to the offensive line.