Flock was well established as a racing figure before NASCAR was formally constituted. He stepped into NASCAR founder Bill France Sr.'s car in 1946 and won both events at the Daytona Beach Road Course in 1947. His approach to competition was characterized by fearless improvisation: during a June 1947 race, Flock overturned his car in an accident but refused to accept a DNF. He enlisted spectators to flip the vehicle back onto its wheels and continued to the finish. In October 1947, a tire failure at Piedmont Interstate Fairgrounds sent his car through the track fencing and into a light pole, leaving him with a crushed vertebra.
The Flock family was also involved in an illegal moonshine business. On one occasion, federal agents staked out an Atlanta race where Flock was competing, intending to arrest him. As the race began, police vehicles pursued him onto the circuit and chased him for several laps before he drove through the fence, eventually running out of fuel. Flock later recalled: "I would have won that race if the cops had stayed out of it."
When NASCAR launched its premier Strictly Stock Series in 1949, Flock qualified on the pole for the very first race at Charlotte Speedway on June 19, setting a time of 38.37 seconds. He won two races that inaugural season and finished third in the points standings behind Lee Petty and champion Red Byron.
Flock remains the only driver to win a Cup Series race at the Air Base Speedway in Greenville, South Carolina, claiming victory there on August 25, 1951. His brothers Tim and Fonty also competed in that event. The track operated for only three years before closing in 1952.
Beyond the premier series, Flock won two 100-lap ARCA events at Lakewood Speedway in 1954, and accumulated more than 200 modified wins across his career. He also started on pole in one NASCAR Convertible Division event at Montgomery Motor Speedway, though a broken axle ended his run in 20th place. His career as a driver ultimately ended when he broke his back in an on-track accident.
Perhaps the most historically notable moment of the Flock family's collective career came at the July 10, 1949 race at the Daytona Beach Road Course. All four siblings — Bob, Fonty, Tim, and Ethel Mobley — competed together. It was both the first NASCAR event to feature a brother and a sister, and the only race in NASCAR history in which four members of the same family competed simultaneously. Ethel finished ahead of both Fonty and Bob, coming home eleventh.
After retiring from driving, Flock became a track promoter in Atlanta. Reflecting his inclusive outlook, he actively recruited women racers for his venue, hiring Sara Christian, Mildred Williams, and his sister Ethel Mobley to compete. Flock died of a heart attack on May 16, 1964, at the age of 46. His impact on the sport's founding years, particularly as part of the legendary Flock family dynasty, left a permanent mark on American stock car racing history.