It was a game in 1980 that first put McMahon in the national spotlight. As a junior at BYU during the 1980 season, McMahon rung up gaudy statistics that were derided by critics as coming against weak opposition. But in the Holiday Bowl that December, McMahon led the Cougars back from a 45-25 deficit in the final 4:07 to stun Southern Methodist University, 46-45. The performance put him on the map, and in following season (1981), McMahon again put up stellar numbers. He finished his college career with 71 NCAA records, and was selected by the Bears in the first round of the 1982 NFL Draft, at number five overall.

McMahon, thrilled to be "released" from what he considered an oppressive culture in Utah, strolled into his first public function with the Bears holding a cold beer in his hand. New head coach Mike Ditka and team founder and owner George Halas were nonplussed. Ever the free spirit, McMahon was to find the atmosphere in Chicago almost as stifling as that at Brigham Young, and he would lock horns with Ditka, his coaches and teammates, and journalists routinely during his career with the Bears.

McMahon won the Bears' starting quarterback job as a rookie and was named to several All-Rookie teams when he nearly led the team to the playoffs, despite the NFL only playing two games before a players' strike that cancelled nearly half the season. McMahon quickly displayed a natural ability to read defenses and an athletic versatility that surprised many. He established himself as the best play-action passer in the game with his nonchalant fake handoffs and coolness in the pocket. Despite having only average arm strength his situational awareness and superior acting skills made him a fearsome play-action passer.

McMahon also made a case for being the best rollout passer at that time. He explained that coaching in his youth had taught him to square his shoulders to the direction he wanted to throw the football, and he was thus able to execute passes with tight spirals and a high degree of accuracy when running to either his left or his right. The Bears finished the strike-shortened season at 3-6, but due to an expanded playoff format and conference-wide seeding the Bears missed a playoff berth by only one victory. McMahon was named NFC Offensive Rookie of the Year, losing the league-wide honor to Los Angeles Raiders running back Marcus Allen, formerly of USC.

In 1983 McMahon continued to improve as a passer and as a field general. He made a habit of changing the play both in the huddle and at the line of scrimmage, a practice which frustrated Ditka but usually led to success. His knowledge of the game and an instinctive, intuitive grasp of in-game situations were significant. He became a frequent scorer in goal line situations after the dying Halas instructed Ditka to make the quarterback sneak a bigger part of the Bears' offense. He also began to catch touchdown passes on option plays, and was the emergency punter. Chicago finished the season at 8-8, missing the division title and a playoff berth by one victory again.

In 1984 the Bears broke through, reaching the conference finals before losing to the San Francisco 49ers. McMahon started the season strongly, though nursing minor injuries like those that would plague him throughout his career. In a violent game against the Los Angeles Raiders in Chicago, McMahon sustained a season-ending injury when he was brutally tackled by two Los Angeles defenders. He suffered bruised ribs and a lacerated kidney on the play, but limped to the huddle and breathlessly called the next play, despite difficulty breathing and increasing pain. The players could barely hear him in the huddle, and when McMahon attempted an audible at the line of scrimmage the Bears receivers were unable to hear his call. McMahon was on the verge of collapsing on the field, clutching his flank and rasping in his attempts to convey his situation. Offensive linemen helped McMahon stand and leave the field. McMahon went to the lockeroom, and reported urine that "looked like grape juice." The gutsy McMahon's season was over, but the admiration in the eyes of his teamates was significant.

In 1985, the Bears won their first 12 games and finished 15-1 for the season. McMahon became a media darling not only for his outstanding play on the field, but also his personality, perhaps best summed up in a rap record made by the team, The Super Bowl Shuffle, in which he proclaims "I'm the punky QB known as McMahon." He ended the season with a strong performance in Super Bowl XX, which was won 46-10 by the Bears over the New England Patriots; McMahon earned a spot in the Pro Bowl. He was a point of controversy in New Orleans at the Super Bowl when he mooned journalists who were inquiring as to the status of a minor injury to his buttocks. McMahon was notorious for head-first baseball-style slides when running the football, despite being coached to slide feet-first to protect his body. In the playoffs McMahon heeded this coaching advice and was speared by a defender's helmet squarely in his buttocks, causing a painful deep bruise that McMahon sought acupuncture treatment for.

No discussion of McMahon's 1985 season would be complete without a mention of an early-season Thursday Night game at Minnesota. McMahon was slated to back up Steve Fuller, as McMahon had missed practice time earlier in the week due to a neck injury that required an overnight hospital stay. Midway into the third quarter the Vikings held a 17-9 lead. McMahon spent much of the second and third quarters pacing alongside Ditka, lobbying to be sent into the game. Ditka quietly dealt with these advances while concentrating on the game, but eventually relented and McMahon, to the excitement of the Bears and the chagrin of the Minnesota fans, entered the game in the third quarter.

With the atmosphere in the Metrodome suddenly charged and electric, McMahon confidently knelt in the huddle and called his first play. The Vikings blitzed at the snap and left Willie Gault open, and McMahon hit the speedy receiver with a 70-yard touchdown pass that stunned the Vikings' players and fans as well as a national television audience. The jubilant Bears were instantly cocky and confrontational, and quickly got the ball back. On McMahon's second play he threw a 25-yard touchdown pass to Dennis McKinnon. McMahon was now 2-2 for 95 yards and two touchdowns. After a Vikings punt the Bears moved across midfield running the football, then McMahon made his third pass attempt of the night, hitting McKinnon again for a 43-yard touchdown. Three passes, three scores, and the Bears cruised to victory.

By 1986 McMahon's reputation as a winner was undisputed, but he was unpopular in many NFL cities due to his sarcastic and pointed comments, and a penchant for brutal honesty. Even some of his teammates had a mild to moderate dislike of the quarterback. He was considered injury-prone and a smart aleck. This negative reputation began to hurt the Bears as defenders singled McMahon out for punishment on the field. In a typically hard-fought and dirty contest against the Green Bay Packers in November McMahon's mouth got him in more trouble than he ever bargained for. Packer's defensive lineman Charles Martin proudly displayed a handtowel in pregame warmups with the numbers of Bears stars listed on it, and claimed it was a "hit list" for the game. Martin was allowed to wear the towel in the game, and after a McMahon interception he sauntered over to the quarterback from behind, grabbed McMahon and body slammed him into the rock-hard artificial turf. Pandemonium ensued as the play was one of the most blatantly illegal in NFL history. Bears players attended to the stunned and injured McMahon as fights broke out and Martin was ejected. The crowd and the Bears players and staff were incensed, and the Packers bravado began to falter. McMahon suffered a separated shoulder which ended his season. The war-like Bears defense showed no mercy on the ensuing drive, as the Packers suffered losses on three consecutive plays, then sustained a blocked punt resulting in a safety. The Bears went on to win the game.

A similar scenario occurred in 1988 at New England when a Patriots defender twisted McMahon's knee and ended his regular season. The play was widely held to have been an intentional and illegal act but no action was taken by the NFL.

He battled injuries for the rest of his career, although at one point between the 1985 and 1987 seasons, he won 23 consecutive regular-season starts. But the Bears never made it back to the Super Bowl, and McMahon was traded to the San Diego Chargers prior to the 1989 season after a falling-out with coach Mike Ditka.

He managed two more full seasons as a starter, in 1991 (with the Philadelphia Eagles) and 1993 (with the Minnesota Vikings), then spent three more years as a backup. He retired following the 1996 season, in which he won a second Super Bowl ring with the Green Bay Packers. His time with the Packers was noteworthy. Despite his history with the Chicago Bears and his unpopularity in many circles he was warmly welcomed in Green Bay He mentored Brett Favre and helped Favre with throwing mechanics. With the Packers stumbling against the Dallas Cowboys at Texas Stadium on a regular basis in big games, Green Bay searched for answers to the puzzle. McMahon helped Favre correct his poor performances in Dallas by explaining that the field at Texas Stadium was dramatically crowned compared to many other fields, and this was why Favre's sideline passes often sailed high.

Throughout his career McMahon was known for both on and off field antics. Most famously his wearing of headbands while on the sidelines, one such led to his being fined by then NFL commissioner Pete Rozelle as it had an un-authorized corporate logo on it. The next week his headband simply said "Rozelle". Reportedly before Super Bowl XX hundreds of fans mailed McMahon headbands in hopes he would wear them during the game and Pete Rozelle gave him a stern warning not to wear anything "unacceptable", in response McMahon decided to help bring attention to Juvenile Diabetes by wearing a headband simply stating "JDF Cure" before switching to one stating "POW-MIA" and finally one with the word "Pluto", the nickname of a friend of his stricken with Cancer.

He is also known for his trademark sunglasses. He wears them for medical reasons; in a childhood accident, a fork was stuck in one eye. While his vision was saved, the accident left that eye overly sensitive to light.

Antics aside, McMahon also had a reputation as a heady quarterback with an uncanny ability to read defenses, and it was this talent that kept him in the game for years after injury had wrecked his physical skills.

Since retiring from football in 1997, he has worked as a restaurant owner and motivational speaker. He was seen playing in a charity golf event at Rich Harvest Links, in Sugar Grove, IL. He was also pulled over in Florida for drunk driving. Upon being pulled over, McMahon allegedly got out of his car and shouted to the police, "All right, you got me."