He led his high school to the 1994 Oregon Class 4A semifinals, becoming wildly touted by newspapers as a college prospect. He also was active on the school track team, where he set a school pole vault record. McNown signed with UCLA after high school. His selection of UCLA was influenced by future NFL quarterback Brock Huard signing with Washington. Huard, along with McNown, were the top high school quarterback prospects in the western United States in 1994.
McNown attended UCLA, and played for the UCLA Bruins football team from 1995 to 1998. He became the starting quarterback as a true freshman, four games into the season, ranking first among all freshmen quarterbacks in many statistics. In 1995, UCLA finished 7–5 and played in the Aloha Bowl (losing to Kansas).
McNown was less successful as a sophomore in 1996, when UCLA would go 5–6 and he would be ranked 9th in the Pac-10 in pass efficiency. But the season ended on a high note as UCLA overcame a 17-point fourth quarter deficit to beat rival USC in overtime, 48–41.
As a junior in 1997, McNown announced the team's goal to score an average of 30 points per game. They ended up averaging 39.75 points per game. After an 0–2 start, UCLA would win its remaining 10 games, including the 1998 Cotton Bowl Classic over Texas A&M, to finish Pac-10 co-champion and ranked No. 5 in the nation. McNown was named Most Outstanding Offensive Player for that year's Cotton Bowl Classic. He also was a finalist for the Davey O'Brien Award, was named an All-American by the Associated Press (third team), The Sporting News (third team), made the All-Pac-10 team (second team) behind Washington State's Ryan Leaf, and finished eighth in the Heisman balloting. He led the nation in passing efficiency with a 168.6 rating. His play broke many UCLA records, most of which had been previously set by Tom Ramsey.
In his senior season in 1998, McNown led UCLA to a 10–2 record, including a Rose Bowl appearance as the sole Pac-10 champion. With McNown at the helm, the Bruins' explosive offense carried them on a school-record 20-game winning streak from the previous year, as they won their first 10 games in 1998, before losing to Miami Hurricanes in their regular season finale in a loss that broke the 20-game winning streak and knocked UCLA out of the BCS Championship Game vs. Tennessee. The disappointed Bruins then lost to Ron Dayne-led Wisconsin in the 1999 Rose Bowl, 38–31. McNown set numerous school records in passing and offense, became the Pac-10's all-time career leader in total offense, and won a collection of post-season honors, including Pac-10 co-Offensive Player of the Year, the Pop Warner Memorial Trophy for best senior player on the West Coast, consensus first-team All-American honors, and the Johnny Unitas Award as the top senior quarterback in college football. McNown also finished third in balloting for the Heisman Trophy. In the 1999 Senior Bowl, McNown earned MVP honors as he threw two touchdowns in helping to lead the South team to victory.
For his career, McNown still holds many of the passing and total-offense records. McNown also holds the distinction of being the only UCLA quarterback to go 4-0 against cross-town rival USC. On October 9, 2009, McNown was inducted into the UCLA Athletics Hall of Fame.
In the months preceding draft day, the Bears had declared that Erik Kramer would be the starting quarterback, but would waive him prior to signing McNown, who they named as the upcoming season's starter. He was a contract holdout most of training camp, eventually agreeing to a $22 million contract.Head coachDick Jauron announced that Shane Matthews would be the starter, but that McNown would play at least one series every game to gain experience.
McNown would start his first game for the Bears on October 10 following a hamstring injury to Matthews the previous week. During the game on December 26, following a poor performance against the Rams, McNown chose to sit out the second half of the game. He would be replaced as starter by Jim Miller, but would once again be named the starter following Miller's suspension. He would finish the season breaking franchise rookie records for completions and passing attempts.
He was named the starter over Matthews (Miller was injured during the pre-season), but his performance grew noticeably worse through the season; the Bears under McNown would fall to 1–6, leading the home crowd to regularly chant for Miller's return. He suffered a shoulder injury during the seventh game of the season (against Philadelphia), was briefly replaced by Miller, who would himself suffer an injury and be replaced by Matthews. McNown would start (and lose) one more game that season, against San Francisco, and was benched for the final game of the season.
McNown was traded during the 2001 preseason to the Miami Dolphins, along with a seventh-round pick, for a sixth-round pick, and a conditional 2003 seventh-round pick. In Chicago, he had fallen down the depth chart, below Miller and Matthews, competing against Danny Wuerffel for the third-string position. He would be named the third-string quarterback for the Dolphins, and saw no action during the season.
The Dolphins traded McNown to the San Francisco 49ers for a conditional seventh-round draft pick during the 2002 offseason. By then, Terry Donahue, former head coach at UCLA, was the general manager. Interest was briefly raised in McNown, as the 49ers were then searching for a quarterback for the west coast offense. Although he was initially competing against Tim Rattay, Giovanni Carmazzi, and Brandon Doman for the backup spot behind starter Jeff Garcia, McNown reinjured his shoulder during the preseason. When it was revealed he would require season-ending surgery, he was placed on injured reserve. McNown was released by the 49ers during the 2003 offseason. His rights were shortly thereafter acquired by the Calgary Stampeders, although he was never signed.