[11] Marshall attended the University of Central Florida, where he played a total of 44 games (21 starts) at wide receiver for the UCF Knights football team.

[6][23] Before the regular season began, Marshall suffered a slight tear to his posterior cruciate ligament (PCL) in the preseason game against the Detroit Lions.

[26] He caught a pass in each of the Broncos' final seven games of the season, totaling 18 receptions, 287 receiving yards, and 1 touchdown over that stretch.

[27] In that game, Broncos quarterback Jay Cutler (who was making his NFL regular season debut as a rookie) threw a pass to Marshall on the right side of the field.

[28] The injury left him out for nearly all of training camp until Broncos head coach Mike Shanahan ordered him to participate for the last couple of days.

[31] On Christmas Eve, 2007, in a Monday night road loss to the San Diego Chargers, Marshall caught six passes for 75 yards.

[39] On March 22, 2008, Marshall slipped on an empty McDonald's bag while wrestling with family members, and subsequently, fell through a television set at his home in Orlando, Florida – cutting his right forearm.

[40][41][42] According to Broncos head athletic trainer Steve Antonopulos, Marshall "sustained right forearm lacerations to one artery, one vein, one nerve, two tendons and three muscles.

[45] Marshall officially received a three-game suspension from the NFL on August 5, 2008, due to off-the-field legal troubles including a drunk driving charge and domestic violence arrest.

[47] In Marshall's first game back from suspension, he caught 18 passes for 166 yards and a touchdown, in a 39–38 home victory over the San Diego Chargers.

[49] Marshall received attention for an attempted touchdown celebration during the Broncos' 34–30 comeback road victory over the Cleveland Browns in a Thursday Night Football matchup on November 6, 2008.

Denver wide receiver Brandon Stokley rushed over to Marshall to urge him to put the glove away, as it could be considered a touchdown celebration prop, which is against NFL rules (a 15-yard penalty on the kickoff could be assessed).

[50] Marshall later explained that he was intending to put on the glove (which was white with one half of it painted black) as a means to honor racial progress and unity in the United States, following the country electing Barack Obama as President two days earlier.

[52] On December 7, 2008, in the 13th game of the season, Marshall caught 11 passes for 91 yards and two touchdowns, as part of a 24–17 home victory over the Kansas City Chiefs.

[60] In a 26–6 Thanksgiving home victory over the New York Giants, Marshall recorded six catches (including two one-handed grabs) for 86 yards.

[62] On December 13, 2009, Marshall broke Terrell Owens' record for receptions in a game with 21, as part of a 28–16 road loss to the Indianapolis Colts.

[69] Marshall was benched for the team's final regular season game by head coach Josh McDaniels for failing to arrive at a physical therapy session on time.

[88] In Week 16 against the Arizona Cardinals, Marshall broke Marcus Robinson's single-season franchise record for receiving yards set in 1999.

[101] In Week 17, Marshall torched the Green Bay Packers by providing an early fourth quarter touchdown from quarterback Jay Cutler.

[114] During Week 14 against the Dallas Cowboys, Marshall was kneed in the back by Barry Church, suffering two broken ribs[115] and a collapsed lung.

[135] Marshall's 2016 season became less productive than the previous year due to mostly dropped balls, constant rotation at the quarterback position, and locker room controversy with teammate Sheldon Richardson.

[7][157] On July 31, 2011, Marshall announced at a press conference that he had been diagnosed with borderline personality disorder (BPD) and that he hopes to spread awareness and understanding about the illness.

On October 10, 2013, Marshall donned a pair of bright green Nike cleats in support of Mental Illness Awareness Week.

[163] On January 1, 2007, Marshall was present at The Shelter, a nightclub in Denver, Colorado, along with teammates Javon Walker and Darrent Williams.

As the players were leaving the club in a limousine, Williams was fatally shot in the neck after an unknown assailant opened fire on the vehicle.

[164] Walker has stated in interviews that the shooter was likely a nightclub patron whose motive was retaliation after being involved in an altercation with Marshall's cousin earlier that night.

[167] In the early morning of October 22, 2007, Marshall was arrested in the Denver-Aurora metropolitan area at the intersection of 14th and Blake St. for driving under the influence of alcohol.

[168] A trial was scheduled for September 16, but Marshall instead agreed to a plea bargain four days earlier; he pleaded guilty to a reduced charge of driving while ability impaired.

[173] On March 1, 2009, Marshall was arrested in Atlanta for disorderly conduct after allegedly being involved in a fight with his fiancée, Michi Nogami-Campbell.

On early Sunday, March 11, 2012, two days before his trade to the Bears, Marshall was accused of hitting a woman in the face at a New York club and being involved in a melee.