He played in the NFL for the Cowboys (winning three Super Bowls as the backup quarterback for Troy Aikman), New York Giants, Tampa Bay Buccaneers, and Miami Dolphins.

Garrett was accepted to Princeton University, where he began as the starting quarterback of the freshman team, registering 64 completions on 116 attempts for 996 yards.

[8] In 1991, Garrett started at quarterback for the San Antonio Riders of the World League of American Football, but he suffered a separated right shoulder in the season opener.

In the eighth game of the season after Aikman injured his left hamstring against the New York Giants in the third quarter, Garrett came in and led two touchdown drives while completing five of six passes for 34 yards.

He was named the starter for the next game against the Phoenix Cardinals, completing two of six passes for 25 yards and helped the team score on a field goal before being replaced on the third series in favor of Bernie Kosar, who had been signed four days earlier.

The highlight of Garrett's playing career occurred in the 1994 Thanksgiving Day game when he started in place of backup quarterback Rodney Peete, who was out with a sprained thumb he suffered in a win against the Washington Redskins.

Garrett led the Cowboys to a 42–31 comeback victory over the Green Bay Packers by completing 15 of 26 passes for 311 yards and two touchdowns in the second half.

On February 27, 2002, Garrett was released for salary cap reasons and re-signed on July 24,[15][16] although he would be declared inactive in 16 games since he was passed on the depth chart by Jesse Palmer, who was later named the backup quarterback.

He guided the Cowboys to the second-best offense in the NFL as the team finished with a 13–3 record,[20] which made Garrett an attractive head coaching prospect.

In January 2008, he interviewed for the head coaching job of the Baltimore Ravens and Atlanta Falcons, receiving offers from both, but ultimately opted to remain with the Cowboys.

[22] On December 29, 2008, the Detroit Lions received permission to speak to Garrett regarding the team's head coaching vacancy according to ESPN sources.

[26] On November 14, Garrett won his first game as a head coach, beating the favored New York Giants by a score of 33–20 at New Meadowlands Stadium.

After Garrett finished the 2010 season with a 5–3 record down the stretch, he was officially named the eighth head coach in Cowboys history on January 6, 2011.

[31] The Cowboys were undefeated on the road, including impressive wins over the Seahawks and the NFC East frontrunners (at the time), the Philadelphia Eagles.

[33] In the Divisional Round against the Green Bay Packers, hyped as a rematch of the famous Ice Bowl NFL championship game from 1967, the Cowboys held a lead in the second half, but after losing the lead due to a late Aaron Rodgers touchdown pass, had a potentially game-winning catch by Dez Bryant that was overturned in official replay, sealing a 26–21 road loss to Green Bay.

[35] The Cowboys entered the 2015 season with great expectations, but starting quarterback Tony Romo played in only four games after suffering two fractures in his left collarbone, and the team finished with a 4–12 record.

[37] However, the Cowboys were upset in their first playoff game by Aaron Rodgers and the Green Bay Packers with a last-second field goal by Mason Crosby, losing 34–31.

[40] This was partially because star running back Ezekiel Elliott was suspended for six games (Weeks 10–15) as a result of violating the NFL personal conduct policy.

[41] In 2018, the Cowboys bounced back from a disappointing 3–5 record at the bye week to win seven of their last eight games and clinch the NFC East for the third time since Garrett became head coach.

[52] * – Interim head coach On April 12, 2022, it was announced that Garrett would be an analyst for NBC Sports broadcasts of the United States Football League.