Al Unser, Jr., the son of Al Unser and the nephew of Bobby Unser, both race car drivers in their own right, grew up surrounded by racing. By the age of 11, Al Junior was racing sprint cars. After high school, he was already in the World of Outlaws series of sprint car racing. He soon moved into road racing, winning the Super Vee title in 1981 and the Can Am title in 1982.

In 1982, Unser made his debut on the CART circuit, finishing fifth at the California 500. A year later, he competed in his first Indianapolis 500, finishing tenth. Unser continued racing on the CART circuit, becoming one of the series' rising stars. He finished second in the CART championship point standings in 1985, losing to his father by just one point. He began competing in the IROC championship in 1986, winning that championship with two victories in four races. At the age of 24, Unser was the youngest IROC champion ever (as of 2004). Unser won the 1988 and 1986 IROC championships. Unser won the 24 Hours of Daytona, also at age 24.

Unser continued to improve on the CART circuit, finishing fourth in the points standings in 1986, third in 1987, second in 1988 and finally winning the series for the first time in 1990. In 1989, Unser was on the verge of winning his first Indianapolis 500, but while battling with Emerson Fittipaldi for the lead, the two touched wheels and Unser spun out, hitting the wall and ending his chances. Unser would have his day at Indy in 1992, however, defeating Scott Goodyear by 0.043 of a second, the closest finish in Indianapolis 500 history (as of 2005).

In 1994, Unser again won at Indy, this time with Penske Racing. His teammate was Emerson Fittipaldi, the man whom he battled with five years before. Unser turned in a dominant season-long performance, winning eight of 16 races on his way to his second CART championship. However, Unser would begin to fade in CART, perhaps due to alcohol problems that would later lead to his arrest. He would finish second to Jacques Villeneuve in CART championship points in 1995, but after finishing fourth in 1996, 13th in 1997, 11th in 1998 and 21st in 1999, Unser would eventually leave CART to join the budding Indy Racing League for the 2000 campaign. Unser won a total of 31 races during his 17 seasons in CART.

He won his first IRL race that same season at Las Vegas. Unser would go on to win a total of three races in his IRL career, but after breaking his pelvis in an all-terrain vehicle accident in October of 2003, Unser had difficulty securing a ride for the 2004 season. He finally signed with Patrick Racing three races into the season, but after a 22nd-place finish in Richmond, Al Unser, Jr. finally announced his retirement from racing on June 30, 2004. Unser continued to remain involved in racing, however, outside of a racing capacity. He served as an adviser for Patrick Racing and worked as a mentor for his son, Alfred Unser, who is currently working his way through the lower ranks in open-wheel racing.

In 2006 Unser announced that he would come back to racing again and he would run the 2006 Indianapolis 500, teamed with fellow former winner Buddy Lazier for Dreyer & Reinbold Racing. This came just days after Michael Andretti also came out of retirement to run the 500. Unser qualified 26th in the 33 car field.