Follmer's rise through American club racing was methodical and thorough. He captured the SCCA United States Road Racing Championship in 1965, establishing his credentials as a road-racing talent capable of competing at the national level. His development through sports car racing gave him the technical grounding that would later allow him to master such radically different disciplines as open-wheel racing, touring cars, and prototype sports cars within the same career window.
He entered the USAC Championship Car series beginning in 1967, accumulating 25 starts across several seasons through 1974. His one oval victory came at Phoenix International Raceway in 1969. He also contested the Indianapolis 500 three times, from 1969 to 1971, with his best result a fifteenth-place finish in 1971 after starting twenty-ninth in the Grant King Racer's turbocharged Offenhauser-powered entry.
The early 1970s represented Follmer's peak, and it was in Can-Am and Trans-Am that he made his most enduring mark. In 1972, he won the Trans-Am championship driving an AMC Javelin, taking four race victories. That same season, he stepped in as a substitute for the injured Mark Donohue at Penske Racing, driving the Porsche 917/10 in the Can-Am series and winning the championship outright. The racing press promptly nicknamed him "George Am." He became the only driver in history to win both the Can-Am and Trans-Am championships in the same year.
Follmer continued in Can-Am through 1974, finishing as vice-champion in both 1973 and 1974 with teams run by Rinzler and Shadow respectively. Across that three-year span he recorded six wins and thirteen podiums in the series.
He returned to Trans-Am competition in 1976 and claimed a second title, this time behind the wheel of a Porsche 934 Turbo, demonstrating that his 1972 championship had not been a one-time circumstance.
At an age when most racing drivers have either peaked or retired, Follmer made his Formula One debut in 1973 with Don Nichols' UOP Shadow team. He took the start of his first Grand Prix at the South African event at 39 years and one month of age, a mark that made him Formula One's oldest debutant since the 1950s — a distinction he has never surrendered. Over thirteen championship rounds in the 1973 season, he scored five points and finished thirteenth in the Drivers' Championship. His best results came in his first two starts: sixth in South Africa and third in Spain, the latter being his only podium in the world championship.
Follmer sampled NASCAR's Winston Cup Series in 1974, contesting thirteen of thirty races and collecting three top-five finishes plus a pole position. That same year and into 1975, he participated in the International Race of Champions, winning a race in the all-star series.
When the Can-Am series was revived in the late 1970s, Follmer returned to it, finishing sixth in 1977 and fifth in 1978. His 1978 highlight came at St. Jovite, where he won in his Prophet-Chevrolet ahead of Alan Jones.
Long after his professional career had wound down, Follmer made a celebrated return at the 1986 24 Hours of Le Mans, co-driving a Porsche 956 to a third-place overall finish — a result that underscored how durable his ability remained even outside his competitive prime.
Away from the track, Follmer operated a Porsche-Audi-Subaru dealership in Pomona, California, later relocated to Montclair, from 1977 to 1990. He continued to compete in vintage racing events for decades after his professional retirement, often driving the same cars he had raced at their period zenith.
His honors reflect the breadth of his career. He was inducted into the Motorsports Hall of Fame of America in 1999, the West Coast Stock Car Hall of Fame in 2019, the SCCA Hall of Fame in 2019, and the Trans-Am Series Hall of Fame in 2025. A limited-edition Saleen Mustang bearing his name was produced in 2013-2014, based on the Boss 302 he had raced in the 1969 Trans-Am season, of which only 250 units were built.