He played college football for the Alabama Crimson Tide, and was selected by the Baltimore Ravens in the first round of the 2014 NFL draft.

As a sophomore, he was credited with 112 tackles, four sacks, and one interception, as the Bobcats finished with a 6–5 record, losing to the Auburn High School Tigers in the opening round of the Class 6A playoffs.

[1] Improving his own school record, Mosley registered 186 tackles as a senior in 2009, also adding seven sacks, five fumble recoveries, and four interceptions.

Theodore finished with a 9–2 season record, yet was upset by future Alabama teammate T. J. Yeldon's Daphne team in the first round of the playoffs.

[17] Mosley led the Alabama football team in tackles (108) for the second consecutive year in 2013, averaging the most tackles-per-game in his college career.

[18] While the Missouri Tigers' Michael Sam is often remembered as the 2013 Southeastern Conference (SEC) Defensive Player of the Year, he and Mosley actually shared the award.

[20] Prior to the NFL Combine, Mosley underwent surgery to repair a labrum issue after dislocating his hip.

Multiple NFL scouts and team representatives attended Alabama's pro day, including head coaches Sean Payton (New Orleans Saints), Marvin Lewis (Cincinnati Bengals), and Chip Kelly (Philadelphia Eagles).

[22] At the conclusion of the pre-draft process, Mosley was projected to be a first round pick by NFL draft experts and scouts.

[23] Mosley attended pre-draft visits and private workouts for multiple teams, but received the most interest from the Buffalo Bills and Baltimore Ravens.

On October 5, 2014, Mosley collected a season-high 15 combined tackles (eight solo), deflected a pass, and made his first career interception during a 21–13 loss at the Indianapolis Colts in Week 4.

He finished his rookie season in 2014 with 133 combined tackles (89 solo), eight pass deflections, three sacks, and two interceptions in 16 games and 16 starts.

The following week, he made ten combined tackles (seven solo) and forced a fumble during a 35–31 loss at the New England Patriots in the AFC Divisional Round.

[38] On September 27, 2015, Mosley recorded nine combined tackles, deflected a pass, and returned a fumble recovery for the first touchdown of his career during a 28–24 loss in Week 3.

Mosley recovered a fumble after teammate Elvis Dumervil stripped the ball from quarterback Andy Dalton and returned it for a 41-yard touchdown in the fourth quarter.

[43] and second-team All-Pro[44][45] He finished the 2016 season with 92 combined tackles (56 solo), eight pass deflections, four interceptions, and a forced fumble in 14 games and 14 starts.

[50] On October 26, 2017, Mosley made four combined tackles, a pass deflection, was credited with half a sack, and returned an interception for a touchdown during a 40–0 victory on Thursday Night Football against the Miami Dolphins in Week 8.

Mosley intercepted a pass by Dolphins' quarterback Matt Moore, that was initially intended for wide receiver Jarvis Landry, and returned it for a 63-yard touchdown in the fourth quarter.

[51] In Week 14, he collected a season-high 14 combined tackles (nine solo) and made a pass deflection in the Ravens' 39–38 loss at the Steelers.

[55][56] On December 30, 2018, Mosley made 4.5 combined tackles (three solo) and a game-winning interception during a 26–24 victory against the Cleveland Browns in Week 17.

Mosley intercepted a pass by Browns' quarterback Baker Mayfield intended for running back, Duke Johnson at 1:06 of the fourth quarter, sealing the victory.

[59] He finished the season as the Ravens leading tackler with 105 combined tackles, along with half a sack, five passes defensed, and an interception.

[62][63] Mosley made his debut with the Jets in Week 1 against the Bills, making 5 tackles and intercepting a pass from quarterback Josh Allen and returning it for a touchdown.