Johncock began racing at tracks around Michigan, including Old Hastings Raceway, Capital City Speedway in Lansing, the Grand Rapids Speedrome, and Berlin Raceway in Marne. He entered USAC and Indy car competition in 1964 driving for Weinberger Racing, running four races before going full-time in 1965. His first USAC victory came at the Milwaukee Mile in August 1965.
In 1966 Johncock left Gerhardt Racing after a winless season and formed his own team, Johncock Racing, with Gilmore Broadcasting as his primary sponsor. He was at that point one of only two owner-drivers in IndyCar, the other being A.J. Foyt. The team won six races between 1967 and 1969. The loss of Gilmore's sponsorship at the end of 1970 sent the operation into decline, and between 1970 and 1973 Johncock went winless while his team closed and he faced bankruptcy.
Johncock's revival began when he joined the STP/Patrick Racing team for the 1973 USAC season. The 1973 Indianapolis 500 was one of the most troubled in the race's history. A multi-car accident at the start involving Salt Walther, combined with two days of rain, pushed the race to a Wednesday afternoon restart. On lap 58, Johncock's teammate Swede Savage was severely injured in a fiery crash. Moments later, pit crew member Armando Teran was struck by a fire truck and fatally injured.
When the race resumed, Johncock took the lead on lap 73 and held it until rain returned on lap 133. As darkness approached, the race was red-flagged and declared complete. The victory celebration was brief and subdued; the banquet was cancelled and Johncock and his crew went to a fast-food restaurant for hamburgers. Thirty-three days after the race, Swede Savage died from his injuries.
Johncock won the USAC national championship in 1976, taking the title from Johnny Rutherford in the final race of the season at Phoenix International Raceway. In 1977 he was leading A.J. Foyt when his crankshaft broke with sixteen laps remaining. In 1979 he won the inaugural race conducted under CART sanction, the Arizona Republic/Jimmy Bryan 150 at Phoenix.
Johncock's second Indianapolis 500 victory in 1982 produced what was at the time the closest finish in the race's history. After falling behind during a final sequence of pit stops, Rick Mears closed rapidly over the final laps, coming from three seconds back in the closing stages to arrive within a car length at the start of the final lap. Johncock's tires were severely degraded, causing heavy understeer through every turn. Mears caught Johncock at the start of the last lap and attempted to pass in Turn One, but Johncock made a decisive defensive move and held on to win by 0.16 seconds. Mears later joked about watching the replay "to see if this time I get around Gordy." Johncock himself acknowledged in later interviews that had Mears possessed a few more years of experience, the outcome might have been different.
Following the 1982 Indianapolis 500 win, Johncock won the 1982 Michigan 500, completing two legs of what was then known as the Triple Crown (Indianapolis, Michigan, and Pocono). He retired from full-time racing in 1985 but returned for occasional appearances in 1987, 1988, 1989, and 1991. His last Indy car win came in the opening round of the 1983 CART PPG Indy Car World Series at Atlanta Motor Speedway, driving a Cosworth-powered Patrick Wildcat.
His final race was the 1992 Indianapolis 500, where he retired with engine failure. Johncock also competed in 21 NASCAR Sprint Cup Series events during his career, recording three top-five and four top-ten finishes.
Johncock was inducted into the Michigan Motorsports Hall of Fame in 1985, the Indianapolis Motor Speedway Hall of Fame in 1997, the International Motorsports Hall of Fame in 1999, the Grand Rapids Sports Hall of Fame in 2000, and the Motorsports Hall of Fame of America in 2002. He was inducted into the United States Auto Club Hall of Fame in 2025.
In 2023, on the 50th anniversary of his 1973 win, Borg-Warner invited Johncock back to Indianapolis for a ceremonial lap. Since his retirement he has focused on a timber business in Michigan. The Speedway provided Johncock with an official Victory Banquet in 2023 to receive the Champion Driver's Trophy he was never properly awarded in 1973 due to the fatalities that shadowed that race.