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 (known as the Miracle Bowl), 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 the 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 unimpressed. 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.

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 for his personality. He appeared in a rap record made by the team, "The Super Bowl Shuffle," in which he proclaimed "I'm the punky QB known as McMahon." He ended the season with a strong performance in Super Bowl XX, which the Bears won 46-10 over the New England Patriots. In that game, McMahon became the first quarterback in the history of the Super Bowl to rush for two touchdowns.[1] 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 for which McMahon sought acupuncture treatment.

In 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 dismay of the Minnesota fans, entered the game in the third quarter.

With the atmosphere in the Metrodome suddenly charged and electric, McMahon knelt in the huddle and called his first play. The Vikings blitzed at the snap and left wide receiver 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 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 by running the football and throwing a few short passes. On a crucial 3rd and short play McMahon, despite his neck injury, got a firstdown on a quarterback sneak play, and the very next play hit McKinnon for a 43-yard touchdown. The Bears led 30-17 and went on to win the game 33-24.

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, which he wears for medical reasons. At the age of six, while trying to untie a knot in a toy gun holster with a fork, he accidentally severed the retina in his right eye when the fork slipped. While his vision was saved, the accident left that eye extremely sensitive to light. On the field, he was among the first to wear a helmet fitted with a tinted plastic visor covering the eyes, leading to nicknames like "Darth Vader" and "Black Sunshine."