[1] Trevathan attended and played college football at the University of Kentucky under head coaches Rich Brooks and Joker Phillips.

[2] In 2010, Trevathan led the Southeastern Conference in tackles with 144 on his way to being named a first-team all-conference by a number of national organizations, including the SEC coaches themselves.

[8] Throughout training camp, he competed for a job as an outside linebacker against Wesley Woodyard, Nate Irving, Mike Mohamed, Cyril Obiozor, Steven Johnson, and D. J.

[9] Head coach John Fox named Trevathan a backup weakside linebacker behind Wesley Woodyard and newly acquired veteran free agent Keith Brooking.

The following week, he made his first career tackle on running back Mike Goodson in the fourth quarter of the Broncos' 37–6 win against the Oakland Raiders.

[15] On January 12, 2013, Trevathan played in his first career playoff game and made one tackle during their 38–35 loss to the Baltimore Ravens in the AFC Divisional Round.

[17] Defensive coordinator Jack Del Rio named him the starting weakside linebacker, opposite Von Miller, to begin the regular season.

[22] He started his first career playoff game and recorded four combined tackles in their 24–17 win against the San Diego Chargers in the AFC Divisional Round.

[23] On August 12, 2014, the Denver Broncos announced that Trevathan had sustained a fracture below his left knee during practice after center Will Montgomery accidentally rolled on to his leg and was expected to miss the first 6–8 weeks of the regular season.

[25] On October 13, 2014, it was reported that the Broncos' had placed Trevathan on their injured reserve with designation to return after he aggravated his fracture under his left knee.

[32] On December 6, 2015, Trevathan recorded two solo tackles, broke up a pass, and returned an interception by Philip Rivers for a 25-yard touchdown during the Broncos' 17–3 victory at the San Diego Chargers in Week 13.

[35][36] The Denver Broncos finished the 2015 season first in the AFC West with a record of 12-4, clinching a first round bye and home field advantage during the process.

[39][40] Trevathan entered free agency as the top inside linebacker available and received major interest from the Tennessee Titans, Atlanta Falcons, and Chicago Bears.

[44] He started the Chicago Bears' season-opener at the Houston Texans and recorded 11 combined tackles (seven solo) and a sack on Brock Osweiler during their 23–14 victory.

[46] On November 28, 2016, the Chicago Bears placed Trevathan on injured reserve for the rest of the season after he suffered a ruptured patellar tendon in Week 12.

[49] He was involved in a controversial play when he delivered a helmet-to-helmet hit with the crown of his helmet to Packers' wide receiver Davante Adams and was flagged for unnecessary roughness.

Many analysts deemed the hit unnecessary due to safety Adrian Amos having Adams wrapped up immediately after he caught a screen pass.

[57] In Week 16 against the San Francisco 49ers, Trevathan made seven tackles and intercepted a pass from Nick Mullens deflected off wide receiver Marquise Goodwin.