Johnson earned Georgia football and baseball letters in 1971, 1972 & 1973. He played quarterback as a sophomore, leading the Bulldogs to an 11-1 record, passing for 341 yards and rushing for 870 yards. In 1973 he passed for 506 yards for the season. He had 431 rush attempts for 1799 yards during his Georgia career becoming one of the top five SEC rushing quarterbacks for a career. He played shortstop for the Georgia baseball team.
He was drafted by the New England Patriots in 1974 in the 5th round of the NFL Draft, pick 112 overall. He played running back with the Patriots his entire pro career from 1974 to 1982. Johnson's best season was in 1976 when he scored 10 total touchdowns and had 1,042 combined yards of rushing and receiving while the Patriots finished 2nd in the AFC East which was good enough for a playoff spot. In his game against the Raiders in the Divisional round, he rushed for 32 yards on 14 carries with one touchdown, though they lost the game 24-21 amid dubious penalty calling. His productivity declined after the 1978 season, a season in which he was one of four players on the team who rushed for over 500 yards (Sam Cunningham, Horace Ivory and quarterback Steve Grogan). Over the next four seasons, he rushed for only 159 yards combined. He played one season with the Boston Breakers of the USFL before retiring. He was named to the New England Patriots All-1970s Team in 2009.
He is a 1996 inductee into the Georgia Sports Hall of Fame and 2000 inductee into the Athens Athletic Hall of Fame. He is vice-president of the Georgia Lettermen's Club.