When he was 16 AC Milan was interested in signing him, but Sochaux's coach Jean Fernandez advised him to stay because mentally he still wasn't strong enough.

[10] Despite being 18, he was inserted into the starting 11, and quickly provided an impact scoring his first professional goal in just his third league match in a 2–2 draw against Stade Rennais.

[16] After announcing his desire to move to another club and only having one year left on his current deal, Sochaux agreed to listen to offers for the player.

Mathieu responded by turning down a contract extension, which effectively allowed him to leave on a free transfer at the end of the 2008–09 season.

[9] On 15 January 2015, he scored his first goal for Barcelona in a 4–0 Copa del Rey away win against Elche (9–0 on aggregate) from a free kick.

[30] Mathieu scored his first La Liga goal for the club against rivals Real Madrid in a 2–1 El Clásico win at Camp Nou on 22 March 2015.

[citation needed] In the 2015 UEFA Super Cup against Sevilla FC in Tbilisi, he conceded a penalty with a foul on Vitolo, which was converted by Kevin Gameiro as the opponents came from 1–4 down to take the game into extra time; Barcelona nonetheless triumphed 5–4.

Upon his return to the Camp Nou, Mathieu netted an own goal for Sporting in a 2–0 loss in their final game of the 2017–18 UEFA Champions League groupstage.

[citation needed] He made his national team debut in a friendly against the United States at the Stade de France on 11 November 2011, playing the full 90 minutes.

[36] Mathieu was named in France's squad for their hosting of UEFA Euro 2016, but dropped out with injury on 28 May, to be replaced by Samuel Umtiti.