Marshall is perhaps best known as a significant member of two Super Bowl championship teams, the 1985 Bears and the 1991 Redskins. In 1985, the Bears, behind one of the most celebrated defenses in league history, finished the regular season 15–1, shut out both opponents in the playoffs, and beat the New England Patriots 46–10 in Super Bowl XX. In a 37–17 week 16 victory over the Detroit Lions, Marshall delivered a stunning hit on Lions' quarterback Joe Ferguson that left Ferguson flat on his back, knocked out cold. But perhaps Marshall's most memorable moment came in the 1985 NFC Championship Game, against the Los Angeles Rams. At the beginning of the fourth quarter, snow began to fall at Soldier Field, eliciting loud applause from the Bears fans in attendance. On the next play, Bears defensive end Richard Dent sacked Rams quarterbackDieter Brock, causing Brock to fumble the football. Marshall picked up the loose football and, alongside William "The Refrigerator" Perry, ran 52 yards through the falling snow. The Bears beat the Rams 24–0, and Marshall's fumble return for touchdown continues to be the highlight from that game most replayed.Fox News Chicago also named that play to be the most iconic moment of the game, and of the season, as well. He also had a good performance in the Super Bowl, recording a sack and recovering a fumble. In 1986, Marshall recorded five interceptions and 5.5 sacks and was named first-team All-Pro for the first time.
In the spring of 1988, Marshall became the first NFL free agent in eleven years to sign with another team, agreeing to a 5-year, $6 million contract offer to play for the Washington Redskins, the team that had eliminated the Bears from the NFL playoffs in each of the previous two seasons. When the Bears declined to match the offer, the Redskins had to give them their two first-round draft picks in the next two NFL Drafts as compensation.
Marshall won another championship ring with the Redskins in the 1991 season, when they beat the Buffalo Bills 37–24 in Super Bowl XXVI, and Marshall finished the game with several tackles and a sack. A week before that, he had a superb performance in the Redskins 41–10 win over the Detroit Lions, sacking Detroit quarterback Erik Kramer three times. Marshall was named second-team All-Pro following the 1991 season and was named first-team All-Pro for the second time in his career following the 1992 season. In 1993, Marshall reunited with Buddy Ryan, who had been the Bears' defensive coordinator during Marshall's first two seasons, signing a contract to play for the Houston Oilers. When Ryan left the Oilers to become head coach of the Arizona Cardinals in 1994, Marshall joined him there for one season. He then finished his NFL career in 1995 as a member of the New York Jets.
In his twelve NFL seasons, Marshall recorded forty-five sacks and intercepted twenty-three passes, which he returned for 304 yards and three touchdowns. He also forced 24 fumbles and recovered sixteen, returning them for seventy yards and two touchdowns. He is among the few players who have recorded twenty sacks and twenty interceptions in their career.
Marshall has spent much of his life after football suffering from injuries he sustained during his professional career. His health has declined as the years progressed, but Marshall has refused to receive surgery to repair his injured spine, shoulder, and knees. Permanently disabled, Marshall’s days of battling other players have been replaced with days of fighting the NFL and the players' union over a settlement pertaining to his injuries.
In 2008, Marshall prevailed in his long-pending dispute over his entitlement to total disability benefits from the Bert Bell/Pete Rozelle NFL Player Retirement Plan. However, by this time, he had filed for bankruptcy. He currently resides in Titusville, Florida.