Cassel filled in capably at quarterback after Tom Brady went down with an injury at the start of the 2008 season, and his reward was a trade to Kansas City, where he received a $63 million contract. In 2009, Cassel passed for 2,924 yards and had 16 touchdown passes and 16 interceptions.
Cassel led the Chiefs to a division title in 2010, passing for over 3,000 yards to go with 27 TD passes and 7 interceptions. He missed time in 2011, passing for 1,713 yards, 10 touchdowns and nine interceptions. He played nine games for the Chiefs in 2012, throwing for 1,796 yards with six touchdowns and 12 interceptions.
He signed with the Minnesota Vikings in the 2013 offseason.