In 1989, at the age of 13, Jason began his career racing in quarter-midgets. After graduation from high school, he began racing midgets full time and just three years later won his first United States Auto Club (USAC) Midget National title. The following year, he not only took the title again, but also scored a Silver Crown National Championship as well.

Leffler didn’t stop there. In 1999, he became the first driver in 37 years to capture three straight USAC National Midget titles. That year, Leffler also made his inaugural venture in the ranks of NASCAR when he entered the Busch Series event at Indianapolis Raceway Park. His success in 1999 led to a Busch Series ride with Joe Gibbs Racing. Leffler started from the pole position
three times, scored 2 Top-5 and 4 Top-10 finishes on his way to a 20th-place ranking in the point standings.

His performance in the Busch Series grabbed the attention of Nextel Cup team owners Chip Ganassi and Felix Sabates, who offered him the seat of the No. 01 for the 2001 season. In his inaugural year at the top level of the sport, Leffler captured his first Bud Pole at Kansas Speedway and finished fourth in the Rookie of the Year battle.

2002 brought a new opportunity for Leffler, who chose to join the ranks of the NASCAR Craftsman Truck Series. Leffler captured an astounding eight poles that year, and posted 11 Top-5’s and 15 Top-10 finishes on his way to fourth place in the Series championship. Leffler scored his first Truck Series win at Dover (Del.) International Speedway in 2003.