He played college football for the USC Trojans and was a fourth-round pick in the 1995 NFL draft by the expansion team Jacksonville Jaguars.
Johnson signed a $25 million contract with the Buffalo Bills the following season and was named starting quarterback by coach Wade Phillips.
Johnson had a tumultuous, injury-ridden run with the Bills and a reputation for frequently being sacked (140 in his career, including 49 in 2000), the inspiration for the nickname "Robo-sack".
Phillips later said that Bills' owner Ralph Wilson had instructed the head coach to start Johnson over Flutie.
Johnson left USC holding virtually every major passing record and spent much of his senior year as a Heisman Trophy candidate.
[17] In Week 7 (October 12), Johnson played his next game of the season, a backup role to Brunell in a 38–21 victory over the Philadelphia Eagles.
[18] Johnson did not play in the Jaguars' only postseason game, a 17–42 loss to the eventual Super Bowl XXXII champion Denver Broncos in the Wild Card round.
First-year Bills' head coach Wade Phillips proclaimed Johnson's arrival as the start of a "new era" for the franchise.
[22] With the Bills facing the Indianapolis Colts in week 5, Johnson suffered a separated rib cartilage during the first quarter and left the game.
[23] Appearing in relief, Flutie led the Bills to a 31–24 victory over the Colts with 24 unanswered points in the second half.
[24] While Johnson was still recovering from his injury, Flutie led the Bills to four consecutive wins before Phillips officially named him the new starting quarterback on November 5, 1998.
[29] Facing the Titans on the road, Johnson played poorly, completing only 10 of 22 passes for 131 yards, taking six sacks, and fumbling three times.
[31] The following game against the Brett Favre-led Green Bay Packers, Johnson again was sacked five times but led the Bills to a 27–18 victory, on 18-for-26 passing for 259 yards.
In the week 6 game against the Miami Dolphins, a 13–22 loss, Johnson was sacked five times and finished 11-for-26 for 178 yards before leaving due to tendinitis.
Johnson returned in Week 11 against the Bears and received some playing time off the bench as Flutie led the Bills to another win.
Despite Flutie's win streak, Johnson was again named the starter heading into the Week 12 game at Kansas City.
Early in the Bills's next game against the Patriots, Johnson received a season-ending injury, and Flutie replaced him for the remainder of the season.
[40] Tim Layden reported for the August 6, 2001 issue of Sports Illustrated that Johnson had the highest sack-to-dropback ratio among quarterbacks who threw at least 190 passes in the 2000 season.
Johnson led a late-game drive, setting up a tie-breaking field goal that gave Buffalo its first win of the season.
[43] Johnson broke his collarbone and left the Week 9 (November 11) game against the New England Patriots late in the fourth quarter, and Alex Van Pelt assumed the starting job for the rest of the 2001 season.
[45] Under new head coach Jon Gruden, the team was looking for a mobile signal caller in the mold of Rich Gannon.
[52] In Week 16 (December 23), King played so poorly against the Pittsburgh Steelers, throwing three interceptions (one of which was returned for a touchdown) versus only five completions, that Rob Johnson started the second half.
[50] On Week 17 (December 29), with a first-round playoff bye on the line, Johnson led the Buccaneers to five field goals against the Chicago Bears at the University of Illinois' Memorial Stadium for the franchise's first-ever victory when the kickoff temperature was below freezing.
[56] The following game against the Bills, Johnson took over in the fourth quarter after Ramsey suffered a hand bruise, to a chorus of boos from the crowd at Ralph Wilson Stadium.
[59] On November 6, 2003, Johnson signed with the Oakland Raiders, who were seeking replacements for injured quarterbacks Rich Gannon and Marques Tuiasosopo.
[60] Succeeding Rick Mirer, Johnson played in the second half of a Week 16 Monday Night Football game on December 22 hosting the Green Bay Packers.
[56] Following his release from Oakland in 2004, Johnson underwent Tommy John surgery, a procedure more commonly performed on baseball pitchers.
[63] In 2012, Johnson joined a series of class-action lawsuits against the NFL contending that the league knew or should have known concussions and repeated head impacts put players at risk of brain disorders later in life.