Beginning in 1926 they competed in the Pikes Peak International Hill Climb, an annual road race held in Colorado.

He began racing in 1957, at age 18, initially competing primarily in modified roadsters, sprint cars and midgets.

His quick pit stops were a factor in the victory, as well as the fact that VPJ had an insurmountable advantage over the field that year.

That season, he won a record ten times on oval, road and dirt tracks to capture the United States Auto Club national championship.

In 1971, with Vel's Parnelli Jones, he won the Indianapolis 500 again, starting from the fifth position with an average speed of 157.735 miles per hour (253.850 km/h), and holding off Peter Revson's McLaren for the victory.

Unser's bid to become the first three-time consecutive Indy 500 champion was thwarted when he finished second to Mark Donohue in the 1972 Indianapolis 500.

By 1977, the team regained competitive form, although Unser would announce his departure for Jim Hall Racing at year's end.

Despite starting the 1978 Indianapolis 500 from the fifth position in a First National City Travelers Checks Chaparral Lola, Unser's car was considered before the race to be a second-tier entry at best, if not an outright long shot to win.

Unser's race average speed of 161.363 mph (259.689 km/h) ranked as the then-second fastest ever run (one mile per hour less than the then-1972 record), and would not itself be topped for second for four more years.

Although Unser went winless for three seasons, he would later say that driving for the Hillin family was his most joyful experience as a driver before his successful Penske years.

In 1986, Penske decided to focus the team's attention on teammate Rick Mears when he healed from serious injuries.

Coincidentally, his son Al Unser Jr. (driving for Shierson Racing) was having difficulty with his car's handling.

Danny Ongais crashed into the wall during the first week of practice, suffering a serious concussion, and was declared unfit to drive.

Midway through the second week of practice, Roger Penske consummated a deal with Al Sr. to drive the third car.

Penske promised Unser a well-funded effort, and a brand new Cosworth engine, the same chassis/engine combination that had won the previous four Indy 500s.

The year-old March was removed from a Penske Racing display at a Sheraton hotel in the team's hometown of Reading, Pennsylvania, and hurriedly prepared for a return to active competition.

On a day when heavy attrition felled most of the field's front-runners, including the overwhelmingly dominant Newman-Haas entry of Mario Andretti, Unser worked his way steadily forward and took the lead on the 183rd lap, after Roberto Guerrero's car stalled on his final pit stop.

Averaging 162.175 mph (260.995 km/h), Unser bested a charging Guerrero by 4.5 seconds to win his fourth Indy 500, only five days before his 48th birthday.

Unser rode the wave of his fourth Indy victory to secure a ride at Penske for the Michigan 500, Pocono 500, and Marlboro Challenge for 1987.

He drove as a substitute for Roberto Guerrero at Nazareth, and had a competitive run until crashing a few laps short of the finish.

With competitive rides filling up, and his career winding down, he joined the sub-par Patrick Racing Alfa Romeo team for 1990.

Instead of jumping into another car "just to make the show," Unser sat out the 1991 race and watched from the sidelines for the first time since 1969.

In 1993, driving for King Racing, he led 15 laps at the Indianapolis 500 to extend his career laps-led record.

A month shy of his 55th birthday, Unser entered the 1994 race with Arizona Motorsports, hoping to qualify for what would be his 28th Indy 500.

[8] Unser continued to suffer from cancer for the next 17 years before dying from the disease aged 82 on December 9, 2021, in his home in Chama, New Mexico.

[15] He also competed in the 1968 Daytona 500 and four other NASCAR Winston Cup & Grand National races, all held on road courses with a best finish of fourth (twice).