During his NASCAR NEXTEL Cup Series rookie season in 2003, Mears competed in all 36 races for Chip Ganassi Racing. He ended the season with five top-20 finishes and placed 35th overall in the point standings. Mears also drove 13 races in the Busch Series, scoring one pole and three top-10 finishes, and ran four ARCA races. Mears won three of those events, giving him his first victory in a stock car.  Casey continued to improve in 2004 and 2005. He turned his late-season 2005 momentum into a fast start in 2006. He became the first full-time NASCAR driver to be part of a winning Grand American Rolex 24 team and finished second to now-teammate Jimmie Johnson in the prestigious Daytona 500. In July 2006, Mears won his first career NASCAR race at Chicagoland Speedway in Busch Series competition.