In college he developed a reputation for causing turnovers, setting a Southeastern Conference (SEC) record with 11 career forced fumbles and earning the nickname "the Honey Badger" after the mammal of the same name.

In his sophomore season, he was recognized as a consensus All-American, won the Chuck Bednarik Award as the best defensive player in college football, and was a finalist for the Heisman Trophy.

After spending a year out of football in 2012, he was selected by the Arizona Cardinals in the third round of the 2013 NFL draft, reuniting him in the defensive backfield with former college teammate Patrick Peterson.

Mathieu has also played for the Houston Texans and Kansas City Chiefs, where he was a member of the team that won Super Bowl LIV.

[5] Mathieu accepted an athletic scholarship to attend Louisiana State University, where he played for coach Les Miles's LSU Tigers football team from 2010 to 2011.

Mathieu earned the nickname "the Honey Badger" for his tenacious ability to play extremely tough football against much larger opponents as well as his dyed patch of blond hair on the top of his head in his college days which resembled the blond patch of hair on the back of a honey badger.

[17] On August 17, 2012, Mathieu withdrew from LSU and entered a drug rehabilitation program in Houston, Texas.

On March 28, 2013, Mathieu attended LSU's pro day, but chose to stand on his combine numbers and only performed positional drills.

[30] (Over 6 years later in a week 17 postgame interview during the 2019 season, Mathieu indicated that he was satisfied that he'd established himself and was okay with the "Honey Badger" nickname.)

[31] Head coach Bruce Arians named Mathieu the backup free safety to begin the regular season, behind Rashad Johnson.

Mathieu made his first career regular-season tackle and forced a fumble on St Louis Rams' tight end Jared Cook.

He forced a fumble before Cook crossed into the endzone for a touchdown and it was recovered by Cardinals' linebacker Karlos Dansby for a touchback in the first quarter.

On September 22, 2013, Mathieu recorded ten combined tackles (nine solo), deflected a pass, and made his first career interception during a 31–7 loss at the New Orleans Saints.

Mathieu intercepted a pass by quarterback Drew Brees, that was initially intended for wide receiver Lance Moore, in the third quarter.

On October 6, 2013, he recorded five solo tackles and made his first career sack during the Cardinals' 22–6 win against the Carolina Panthers.

[36] On December 8, 2013, Mathieu made one solo tackle and a pass deflection before exiting the Cardinals' 30–10 victory against the Rams with a leg injury.

[38] He completed his rookie season in 2013 with 68 combined tackles (65 solo), nine pass deflections, two interceptions, a sack, and a forced fumble in 13 games and 11 starts.

Upon his return, head coach Bruce Arians named Mathieu the backup free safety behind Rashad Johnson.

On November 2, 2014, Mathieu recorded five combined tackles, a pass deflection, and made his second career interception during a 28–17 win at the Dallas Cowboys in Week 9.

[43] On November 30, 2014, Mathieu made three combined tackles in the Cardinals' 29–18 loss against the Atlanta Falcons, but exited in the third quarter after sustaining a thumb injury.

[44] Mathieu finished the season with 39 combined tackles (35 solo), four pass deflections, an interception, and a fumble recovery in 13 games and six starts.

[45] Defensive coordinator Todd Bowles accepted the head coaching position with the New York Jets and was replaced by James Bettcher.

[49][50] On December 20, against the Philadelphia Eagles, Mathieu tore his right ACL after an interception, causing him to miss the final two games of the regular season in addition to the post-season.

[73] In Week 13 against the Denver Broncos on Sunday Night Football, Mathieu intercepted two passes thrown by Drew Lock.

[74] Overall, Mathieu finished the 2020 season with 62 total tackles, nine passes defended, one fumble recovery, and a career-high six interceptions.

[77] In the Divisional Round of the playoffs against the Cleveland Browns, Mathieu intercepted a pass thrown by Baker Mayfield during the 22–17 win.

[88][89] In the 2022 season, Mathieu started all 17 games, with one sack, 91 total tackles (64 solo), three interceptions, eight passes defended, and one forced fumble.