Harris played college football for the Grambling State University Tigers from 1965 to 1968. Under the guidance of head coach Eddie Robinson, Grambling won or shared all four Southwestern Athletic Conference (SWAC) titles while Harris was a player and he was named MVP of the 1967 Orange Blossom Classic.
Harris was drafted in the eighth round of the 1969 Common Draft by the American Football League's Buffalo Bills, and would soon join fellow rookie O.J. Simpson in the starting backfield. Continuing the American Football League's more liberal (than the NFL's) personnel policies, the Bills made Harris the first black player to start a season at quarterback in the history of pro football. Harris was also just the second black player in the modern era to start in any game as quarterback for a professional football team. Wide receiver Marlin Briscoe, of the AFL's Denver Broncos, had been the first to start a game at quarterback in 1968, and a few of Harris's completions in 1969 went to Briscoe, who, by that time, had been traded to the Bills and had been converted to the position of receiver. After three years with the Bills, Harris was released by the team and signed by the Los Angeles Rams in 1972.
In 1973, Harris was the understudy to veteran John Hadl as the Rams went 12-2 and returned to the playoffs for the first time since 1969 . As the 1974 season began, the Rams offense sputtered under Hadl and the team stood at 3-2 after five games. In an effort to spark the Los Angeles offense, Rams head coach Chuck Knox promoted Harris as the starting quarterback. In his starting debut for the Rams against the San Francisco 49ers, Harris completed 12 of 15 passes for 276 yards and three touchdowns and rushed for another as the Rams won easily, 37-14, at the Los Angeles Coliseum. Two days later, Hadl was then traded to Green Bay, and Harris became the Rams' first-string quarterback for the remainder of the 1974 season. The football world was stunned by the bold move. However, Harris came through by leading the team to seven wins in its last nine regular season games. He led the team to its second straight NFC Western Division title, and their first playoff victory (19-10 over the Washington Redskins) since 1951. Harris thus became the first African-American quarterback to start and win an NFL playoff game. The Rams lost the NFC Championship Game to the Minnesota Vikings 14-10, as the Vikings were aided by some controversial officiating. Harris was named to the NFC Pro Bowl team in 1974 and was awarded MVP of that game.
The strong-armed Harris was somewhat stymied by Ram coach Knox's conservative, "run-first" offensive philosophy, but still helped lead the team to another division title in 1975. However, he injured his shoulder very early in the Rams' Week 13 win over the Green Bay Packers; backup Ron Jaworski then led the Rams to wins against Green Bay and the Super Bowl Champion Pittsburgh Steelers, as well as to a 35-23 win over the St. Louis Cardinals in the divisional playoff game. Stating that a "player cannot lose his starting job due to injury", Knox named Harris the starter for the NFC Championship games vs. Dallas, as he appeared to be recovered from his injury. Harris' first pass was intercepted, and after one more incompletion and a Dallas 21-0 first quarter lead, he was pulled in favor of Jaworski. It didn't matter as Dallas went on to a 37-7 win.
Harris' injuries continued to give him problems in the 1976 season. The Rams went with three quarterbacks; Harris, Jaworski, and rookie Pat Haden from USC. With Harris injured, Jaworski opened the season as the starter and was injured in the opener. Haden led the team to a comeback tie against the Minnesota Vikings in the second game. Harris, with his throwing shoulder mended, reclaimed his starting job and led the team to two wins, including a 436-yard passing performance against the Miami Dolphins. However, in the next game, on Monday night at home against the San Francisco 49ers, Harris was sacked 10 times and re-injured his shoulder as the Rams were shut out 16-0 at home for the first time since moving to Los Angeles. The Jaworski/Haden platoon led the team to two more wins, then Harris returned again for a win over the then-first-year Seattle Seahawks and a loss to the Cincinnati Bengals. Against the Bengals, however, Harris played poorly, and Knox was forced by Rams owner Carroll Rosenbloom to go with Haden as the starter for the rest of the season. The Rams won three out of four and the NFC West division title in spite of the change. Knox was forced to go with Haden in the 1976 playoffs, even though Harris was healthy enough to see action late in the season, including a season-ending comeback win over the Detroit Lions. However, Knox had to restrict the Ram offense to compensate for Haden's inadequacies ( Haden was 5-10 and could barely throw the ball fifty yards downfield). The Minnesota Vikings took full advantage of this in the NFC title game and easily beat the Rams 24-13 to advance to Super Bowl XI. Ironically, despite his benching, Harris finished the season as the NFC's top-rated passer. It was the first time a black quarterback ever led his conference in that category.
Harris was traded to the San Diego Chargers prior to the 1977 season. The experience with the Rams hurt him. "I lost my passion," he said in the book Third and a Mile. "Coach Knox was supportive but the owner (Rosenbloom) was going over his head.
"As a quarterback, I had done all I could, more than most people could, but it still wasn't enough for the Los Angeles Rams organization to accept me as a quarterback, not a black quarterback," he commented. The Rams acquired the aging Joe Namath (who eventually played only four games for the team) and went with the limited Haden again and once again were eliminated by the Vikings at the Los Angeles Coliseum in December 1977 as Haden's hands were too small to grip the football in the rain of that soggy afternoon. When he left the Rams, Harris had the highest career completion average of any quarterback in team history (55.4%) and had been an integral part of three straight NFC West Champions.
Harris was understandably jolted by the trade from a perennial playoff team to a team in rebuilding mode. He had lost his first-string position with the Rams and he was traded to a team that already had a young first-string quarterback in Dan Fouts around whom they were building. Harris played in San Diego for three years, starting a total of 11 games, primarily when Fouts was a contract holdout in 1977, or was injured.
Harris served as the Baltimore Ravens pro personnel director from 1997 to 2003. He served as the vice president for player personnel for the Jacksonville Jaguars. He resigned on December 23, 2008. He also served on the NFL subcommittee on college relations.
On February 2, 2009, the Detroit Free Press reported that the Detroit Lions were set to hire Harris as a personnel executive. On February 12, 2009, the Detroit Lions officially named Harris as Senior Personnel Executive. Lions General Manager Martin Mayhew has a long history with Harris, and indicated he was the only individual who was offered the job. Harris assisted in all areas of player personnel in an advisory role. He is no longer employed by the NFL. He officially "retired from the NFL" on February 27, 2015.
Harris has been inducted into the Southwestern Athletic Conference Hall of Fame, the Grambling Athletic Hall of Fame, the Louisiana Sports Hall of Fame and the Black College Football Hall of Fame.