He played college football for the Michigan State Spartans and was selected by the Washington Redskins in the fourth round of the 2012 NFL draft.

He broke his ankle in his junior year and was forced to attend camps in order to make an impression with recruiting colleges.

[17] Cousins was named second-team All-Big Ten by the coaches, and played his final game as a Michigan State Spartan on January 2, 2012, defeating the Georgia Bulldogs in the Outback Bowl.

[26] Cousins was viewed as an insurance policy in case Griffin was injured, with Redskins head coach Mike Shanahan saying "You're one or two plays away from being the starter...And if I see that value out there on the third day of the draft, I'm going to take that.

[30] In the fourth and final game of preseason, against the Tampa Bay Buccaneers, Cousins completed 15 of 27 passes for 222 yards as the Redskins won 30–3.

Cousins went 2–2 and finished the drive with an 11-yard touchdown pass to Pierre Garçon with 29 seconds remaining, to bring the Redskins within 2 points of the Ravens.

On the two-point conversion, Shanahan called for a run-pass option, and Cousins scored on a quarterback draw to tie the game at 28.

[47] The following week against the New York Giants, Cousins had a poor showing, throwing for a touchdown with four interceptions and a lost fumble.

[51] On October 4, Cousins led a 90-yard game-winning drive against the Eagles, his first since 2012, when he threw a touchdown pass to Pierre Garçon with under a minute left in the fourth quarter, winning the game 23–20.

[54] Cousins and his brother Kyle applied to trademark the catchphrase later in the season and began to sell T-shirts with the phrase to raise money for the International Justice Mission charitable organization.

[59] Against the New Orleans Saints on November 15, Cousins threw for a career-high four touchdowns, as well as posting a perfect passer rating of 158.3, making him the first Redskins quarterback since 1950 to do that with at least 20 attempts.

The performance led to the Redskins' victory over the Eagles with a score of 38–24, which allowed the team to win the NFC East division title for the first time since Griffin III did it in 2012.

The tag acted as a one-year, $20 million contract, which prevented other teams in the league from signing him without giving up two first round draft picks to the Redskins.

[86] During the game, Cousins played throughout most of the second half, and notably forced a fumble on Denver Broncos cornerback Aqib Talib after chasing him down the field after throwing an interception late in the fourth quarter.

[100] Two weeks later against the Los Angeles Rams on Thursday Night Football, Cousins threw for 422 yards and three touchdowns in the 38–31 road loss.

[102] The Vikings won their next two games, against the Miami Dolphins and Detroit Lions, with Cousins passing for five touchdowns and an interception combined.

[105] The Vikings entered the 2018 season with strong expectations fresh off an appearance in the NFC Championship and Cousins as Quarterback, but ultimately missed the playoffs.

[121] In the NFC Wild Card Round against the Saints, Cousins threw for 242 yards and the game winning touchdown to tight end Kyle Rudolph in overtime during the 26–20 road victory.

[127] The following week against the Houston Texans, Cousins threw for 260 yards and a touchdown during the 31–23 road victory, the Vikings' first win of the season.

[135] Three weeks later against the Saints on Christmas Day, he threw for 291 yards and three touchdowns during the 33–52 road loss, eliminating the Vikings from postseason contention.

[150] In the following week against the Eagles on Monday Night Football, Cousins passed 27 completions of 46 attempts for 221 yards with one touchdown and three interceptions in a 7–24 blowout loss.

[174] Against the Los Angeles Chargers in Week 3, Cousins completed 32-of-50 passes for 367 yards, three touchdowns, and a late interception in the end zone that sealed a 28–24 defeat.

[175][176] In the following week against the Panthers, Cousins overcame two interceptions, including a 99-yard pick six, and threw two touchdown passes to Justin Jefferson in a 21–13 victory.

[177] Against the defending Super Bowl champion Kansas City Chiefs in Week 5, Cousins completed 29 passes of 47 attempts for 284 yards and two touchdowns in a 27–20 loss.

[180] In Week 7 against the 49ers on Monday Night Football, Cousins took no sacks and completed 35 passes out of 45 attempts for 378 yards, two touchdowns, and an interception in a 17–22 victory.

[181][182] During the October 29, 2023, game against the Packers in Week 8, Cousins threw two touchdowns before suffering a season-ending torn Achilles injury during the fourth quarter.

[187] A day after his signing became official, reports began to surface that the Falcons were suspected to have tampered with Cousins, which prompted the league to launch an investigation.

[197][198] In Week 15, he passed for a season-low 112 yards and an interception against the Las Vegas Raiders but ended his touchdown drought in a 15–9 win that snapped Atlanta's losing streak.

[199] The following day, head coach Raheem Morris announced that Penix would replace Cousins as the starting quarterback, beginning in Week 16.

[217] During his time with the Redskins, Cousins earned the nickname "Captain Kirk" from the media for his presence as a leader, referencing the Star Trek character.