He joined the circuit full-time in 1970 with
Pit Crew Chief, John Hill. He had 23 top-10 finishes in 45 races, a pole at
Langley Field Speedway, and finished eighth in the points. He raced in the #72
L.G. DeWitt/DeWitt Racing car.
Parsons had 18 top-10 finishes in 35 starts in 1971, including his first win at
South Boston Speedway. He finished eleventh in the points.
In 1972 he had 19 top-10 finishes in 31 races. He finished fifth in the final points standings.
In 1973 he won the
NASCAR Championship with only one win, even though
David Pearson won eleven races (but Pearson only entered eighteen events). Parsons consistency likely won him the championship: he had 21 top-10 and 15 top-5 finishes in the 28 events. His improbable return to the track after an early crash cemented his championship at Rockingham, North Carolina. He saw his championship hopes start to fade as he was involved in a lap 13 crash and his car was heavily damaged. He took to the pits to muster whatever he could out of the car and hope for a top five finish in the final standings. The rest of the garage was hoping to see the underdog unseat the mighty
Richard Petty and joined in to help Parsons' crew put the car back together. Parsons miraculously got back on the track 136 laps later and completed enough laps to finish 25th and take the 1973 championship. Richard Petty, with the championship in his sights after winning the pole and seeing Parsons' accident, had engine trouble and was relegated to a 35th place finish. The poor performance dropped Petty all the way to fifth in the final standings, as Cale Yarborough took the runner up spot on the season with his third place effort. He finished 67.15 points behind the champion. Cecil Gordon's 11th place finish was good enough for third in the standings and James Hylton finished 19th, enough to overtake Petty for fourth.
1973 is considered the start of the modern era in NASCAR, so Parsons is considered the first modern era champion. Parsons also became the only person to win both ARCA and NASCAR championships.
Parsons finished between third and fifth in the final points from 1974 to 1980. He won the 1975
Daytona 500. He switched to the #27 car for
M.C. Anderson starting in 1979.