In 1991, Stewart got a scholarship to the University of Colorado. Playing under coach Bill McCartney, he became one of the most prolific quarterbacks in school history, setting several Buffaloes records, including most passes completed, most passing yards, and most touchdown passes.
Stewart led the option-oriented Buffaloes to a pair of top-10 regular-season finishes in the AP and Coaches polls in 1992 and 1994 as well as wins in the 1993 Aloha Bowl and 1995 Fiesta Bowl. He was selected as a second team All-American his senior year as nationally ranked No. 3 Colorado finished 11–1 in 1994.
One of his greatest moments ever for the Colorado Buffaloes came at the very beginning. On September 5, 1992, Stewart started his first game as a sophomore as Colorado unveiled a pass-oriented offense that lifted the 12th-ranked Buffaloes past in-state rival Colorado State, 37–17. Stewart connected on 21 of 36 passes for a then school-record 409 yards and four touchdowns in the contest.
On September 24, 1994, Stewart threw the game-winning, 64-yard Hail Mary touchdown pass into the end zone to wide receiver Michael Westbrook in the closing seconds to beat the Michigan Wolverines.
Stewart got his chance to start at quarterback in 1997. Prior to that, with Neil O'Donnell as the starting quarterback, he played wide receiver. In his first season as a starting quarterback in the NFL, he led the Steelers to an 11–5 record and the AFC Championship Game which they lost to the Denver Broncos. Stewart threw three interceptions (two of which were made in the opposing Denver end zone), and lost a fumble accounting for all four Steeler turnovers in a 24–21 loss.
Prior to the 1998 season, the Steelers lost their offensive coordinator, Chan Gailey. To replace him, the Steelers brought in Ray Sherman from the Vikings. In addition, Stewart's leading receiver and Pro Bowler Yancey Thigpen also left via free agency after the 1997 season. By mid-season in 1998, the Steelers had lost three of the five starting offensive linemen from the 1997 AFC Championship game and, as a result, Stewart and the Steelers offense struggled and failed to make the playoffs in both seasons.
Coach Bill Cowher named Kent Graham quarterback to start the season, and the team got off to a 1–3 start. When Graham injured his hip, Stewart was tapped to replace him. The team finished 9–7 and barely missed the playoffs to the New York Jets, who had lost to the Steelers with Stewart as the quarterback.
As starting quarterback, Stewart led the 2001 Steelers to a 13–3 regular-season record and the top seed in the AFC playoffs. Under the tutelage of new QB coach Tom Clements and new offensive coordinator Mike Mularkey, Stewart had his best year as a pro, throwing for over 3,000 yards, completing 60 percent of his passes, and attaining a passer rating of 81.7. He threw for 14 TDs and ran for five more. Stewart was elected to the Pro Bowl and was named the Steelers MVP.
The Steelers easily defeated the then-defending Super Bowl champion Baltimore Ravens in the divisional playoffs. The eventual Super Bowl champion New England Patriots defeated the Steelers in the AFC Championship Game.
Stewart began the 2002 season as the Steelers' starting quarterback, but after throwing an interception into double coverage in the end zone at home against the Cleveland Browns in the third game of the season, he was replaced by increasingly popular backup Tommy Maddox, and Stewart was released at the end of the season. Stewart did start two games in place of an injured Maddox to keep the team's playoff hopes alive. Maddox returned, and Stewart never played in Pittsburgh again.
The following season, Stewart signed as a free agent with the Chicago Bears and was named the starter. After several poor performances in 2003, he was replaced in favor of Chris Chandler. After getting the same results from Chandler, he was reinserted as the starter, before getting benched in favor of rookie Rex Grossman after the Bears were eliminated from the playoffs. Stewart was released at the end of the season.
Stewart was signed in 2004 by the Ravens to play a backup role to Kyle Boller, and later served as a replacement for the injured Anthony Wright. He did not, however, throw a pass that season. He was unexpectedly successful as an emergency replacement for punter Dave Zastudil, being named NFL Special Teams Player of the Week for his performance. He was released following the 2004 season, but due to an injury to starting quarterback Kyle Boller during Week #1 of the 2005 season, the Ravens once again signed Stewart to be a backup to Anthony Wright.
Pittsburgh Steelers color commentator Myron Cope gave Stewart the nickname "Slash" to acknowledge that he could play multiple positions—quarterback/receiver/rusher (quarterback "slash" receiver "slash" rusher).
After confining him to many utility roles during the 1995 season, when the Steelers made it to Super Bowl XXX only to lose to the Dallas Cowboys, the team gave Stewart the opportunity to compete for the starting quarterback position in 1996. Following a preseason battle, Bill Cowher named Jim Miller as the Steelers' starting quarterback and kept Stewart in his "Slash" role. Miller struggled at Jacksonville on opening day and was replaced by Mike Tomczak for the rest of the season. In a December 1996 game against the Carolina Panthers, Stewart set a then-NFL record for longest touchdown run by a quarterback with an 80-yard rush.
Following Boller's reactivation on November 9, 2005, the Ravens cut Stewart instead of keeping him as a third-string player, activating running back Musa Smith instead. Stewart had no comment following the announcement he had been cut from the team.
Stewart made guest appearances on the TV shows Deal or No Deal and Pros vs. Joes.
On April 29, 2008, in an interview on the WCNN "680 The Fan", Stewart expressed interest in returning to the NFL. Also, according to ESPN2's First Take on July 28, 2008, Stewart had not officially retired and had been working out at his home, to attempt a comeback during following pre-season. He later was put into free agency but was never selected by a team.
He appeared in January 2009 as an analyst on ESPN's College Football Live, and then later appeared again in July during the show's "50 States Tour" for the show's episode regarding Colorado athletics.
Stewart was also a sideline reporter for the United Football League and is currently an analyst on various ESPN shows such as NFL Live, NFL 32, Take 2, and Mike and Mike in the Morning.
Even though he hadn't attempted a pass in nearly nine years, on May 30, 2012, Stewart officially retired from the NFL.
Stewart has a son: Syre, with ex-girlfriend Tania Richardson.
Stewart met Porsha Williams, a granddaughter of civil rights leader Hosea Williams, in 2009 at downtown Atlanta's Luckie Lounge. On May 21, 2011, they married in a lavish ceremony that was featured on WE tv's Platinum Weddings. Stewart filed for divorce from Williams on March 22, 2013, on the grounds that the marriage was irretrievably broken. Initial reports stated he was unwilling to pay any spousal support.
In October 2012, Stewart began his current position as an afternoon drive radio sports talk show co-host with Atlanta's WZGC "92.9 The Game". As of September 12, 2015, Stewart is not listed as "on-air" talent anywhere on WZGC's website.
Stewart is an avid golfer and carried a nine handicap when he was an active NFL player.