[1] Born and raised in Las Vegas, Nevada, Marshall attended its Cimarron-Memorial High School, where he played football and was also a member of the track & field team.

[10] Head coach Mike Mularkey named Marshall the backup left outside linebacker, behind Daryl Smith, to begin the regular season.

[21] Upon joining the active roster, head coach John Fox named Marshall the backup strongside linebacker behind Nate Irving.

On January 12, 2014, Marshall appeared in his first career playoff game as the Broncos' defeated the San Diego Chargers 24–17 in the AFC Divisional Round.

On February 2, 2014, Marshall appeared in Super Bowl XLVIII and recorded one solo tackle as the Broncos lost 43–8 to the Seattle Seahawks.

[23] Throughout training camp, Marshall competed to be a starting outside linebacker against Steven Johnson, Lamin Barrow, Corey Nelson and Shaquil Barrett.

[26][27][28] He made his first career start in the Broncos' season-opener against the Indianapolis Colts and recorded nine combined tackles (seven solo) and a pass deflection in their 31–24 win.

Marshall sacked quarterback Alex Smith for a five-yard loss in the fourth quarter and forced a fumble by running back Knile Davis.

[30] In Week 14, Marshall recorded nine solo tackles, a season-high three pass deflections, and made his first career interception in the Broncos' 24–17 win against the Buffalo Bills.

Marshall intercepted a pass attempt by quarterback Kyle Orton, that was intended for tight end Scott Chandler, in the third quarter.

On January 11, 2015, Marshall started in his first career playoff game and made six combined tackles in their 24–13 loss to the Colts in the AFC Divisional Round.

[34] On January 28, 2015, the Broncos hired Wade Phillips as their new defensive coordinator after Jack Del Rio accepted the head coaching position with the Raiders.

[37] He started in the Broncos' season-opener against the Baltimore Ravens and recorded seven combined tackles, deflected a pass, and sacked quarterback Joe Flacco in their 19–13 victory.

Marshall intercepted a pass by quarterback Ben Roethlisberger, that was intended for running back DeAngelo Williams, in the fourth quarter with the Broncos losing 34–27 with two minutes remaining.

[42][9] On September 8, 2016, Marshall became the third NFL player to join in protest of American police brutality and racism by kneeling during the national anthem before the season opener against the Panthers.

[44] On September 14, 2016, in a Super Bowl 50 rematch in the 2016 regular season opener, Marshall was fined $24,309 for a helmet-to-helmet hit on Panthers' quarterback Cam Newton.