Leonard won the inaugural AMA Grand National Championship Series in 1954 and captured the title again in 1956 and 1957. His record in the series included 27 wins and the 1957 and 1958 Daytona 200. He also finished as vice-champion in 1958, 1960, and 1961, confirming his status as one of the era's most consistently competitive riders. Leonard retired from motorcycle racing at the end of the 1961 season and turned his attention entirely to automobile competition.
Leonard made his transition to four wheels through the NASCAR super modified circuit running at Fresno and San Jose before debuting in USAC National Championship racing during the 1964 season. He drove for various teams in five races that year, earning a best finish of fifth. In 1965, he became teammates with Dan Gurney at All American Racers and earned his first championship car win at the Milwaukee 150.
Through the mid-1960s, Leonard steadily built his reputation across pavement ovals, road courses, and dirt tracks, earning top-five finishes in the USAC point standings. His first Indianapolis 500 top-ten came in 1966 with a ninth-place finish.
Leonard's most memorable single race came at the 1968 Indianapolis 500, driving the STP Lotus 56 — a turbine-powered wedge-shaped car run by the Granatelli team. He qualified on pole position at a new track record of 171.599 mph and led 31 laps of the race, battling Bobby Unser and Lloyd Ruby. He was in the lead under a yellow flag with fewer than ten laps remaining when the race restarted on lap 191. Moments into green-flag racing, Leonard's fuel shaft broke, ending his race in 12th place. His teammate Art Pollard suffered the identical mechanical failure almost simultaneously. Bobby Unser won. The near-miss remains one of the defining heartbreak stories of the turbine car era at Indianapolis.
Leonard's career reached its peak driving for Vel's Parnelli Jones Racing under legendary chief mechanic George Bignotti. In 1971, driving the PJ Colt-Ford, Leonard delivered consistent results throughout the season, winning the California 500 at Ontario Motor Speedway and clinching the USAC National Championship at the penultimate race of the year.
In 1972, as part of a formidable three-car lineup alongside Al Unser and Mario Andretti, Leonard delivered his most dominant championship season. He won the Michigan 200, the Pocono 500, and the Tony Bettenhausen 200 in three consecutive races, then clinched his second straight title at the California 500. The back-to-back championships placed him among the elite of the USAC era.
Leonard's 1973 season was a struggle, the Vel's Parnelli Jones team unable to match new designs from competing operations. In 1974, a rear tire failure at Ontario Motor Speedway sent his car across the track and into both the pit wall and the outside barrier, leaving him with a compound fracture of his left ankle and a gash above his eye. Because the injury occurred before modern orthopedic reconstruction techniques had advanced through motorsport medicine, he was left with lasting physical limitations. A 1975 comeback attempt ended when he failed the USAC physical examination.
In 1991, Leonard was inducted into the Motorsports Hall of Fame of America in the Motorcycles category. He was inducted into the AMA Motorcycle Hall of Fame in 1998 and the San Jose Sports Hall of Fame in 2001. Legendary mechanic and team owner Smokey Yunick, who worked with Leonard in 1969, wrote of him: "If there was such a thing as a natural born racer, he was it. He liked to go fast, very fast and knew how to do it."