[citation needed] Mahut had excellent junior results, winning the Orange Bowl in 1999 and the Wimbledon Boys' Singles in 2000, turning professional the same year.

Mahut is known for being part of the longest match in professional tennis history against John Isner in the first round of the 2010 Wimbledon Championships.

He has won doubles titles with countrymen Julien Benneteau, Arnaud Clément, and Édouard Roger-Vasselin, before his most successful and current partnership with Pierre-Hugues Herbert.

The highlight of his season was making the doubles semifinal at the 2004 US open, losing to eventual champions Mark Knowles & Daniel Nestor.

[7] His continued success with Julien Benneteau saw them reach new heights at the Paris masters, collecting the runner-up to Rohan Bopanna & Aisam-ul-Haq Qureshi.

Mahut had to qualify for the event but went on to win his first ATP singles title, with a straight sets victory over Stanislas Wawrinka in the final.

Mahut's good form continued into next week, where he participated in Rotterdam with Llodra, and won the tournament while only dropping a set.

The next week at the Miami Masters, Mahut and Llodra reached the semifinals, again they were defeated by the Bryan Brothers, also in straight sets.

The week after Wimbledon, he partnered Sergiy Stakhovsky at the Hall of Fame Tennis Championships, and reached the semifinals where they were defeated in straight sets.

For the remainder of the season Mahut failed to win consecutive tour-level doubles matches, but won 2 Challengers near the end of the season, at Mons, Belgium, partnering Gicquel, without dropping a set in the process, and at Mouilleron Le Captif, France, partnering Pierre-Hugues Herbert and did not drop a set en route either.

In doubles, Mahut and Herbert were seeded 10th and reached the third round before Matkowski and Zimonjić avenged the loss earlier during the grass season.

As a result, Mahut became the first, and the only player to date, to defend a title in both singles and doubles on the ATP World Tour since the new format started in 2009.

However, the streak came to a stop when they entered the Rio Olympics representing France and was taken out in the first round by unseeded Colombians Juan Sebastián Cabal and Robert Farah.

Mahut and Herbert teamed up for the doubles rubber, but were upset by Marin Čilić and Ivan Dodig in 4 closely contested sets.

In singles, Mahut was seeded 7th, but was upset by unseeded eventual runner-up Argentine Diego Schwartzman in 2 tight sets in the first round.

In the semifinals, the duo faced 8th seeds Rohan Bopanna and Daniel Nestor and again prevailed in a 3-set match to reach his second Paris Masters final.

In the final, despite being the favorites on home soil, they lost to the unseeded pair Henri Kontinen and John Peers in another 3-set match.

Mahut and Herbert made their second straight ATP World Tour Finals appearance to wrap up the season, this time seeded 1st.

The pair was upset by Klaasen and Ram in straight sets in the first match in a rematch of 2016 Miami Masters Men's Doubles final, and was 2 points away from winning the second match against López and López in the decider tiebreak in a rematch of 2016 Paris Masters Men's Doubles semifinals, therefore eliminated again at round robin stage.

In contrary, 2nd seeds Jamie Murray and Bruno Soares won all of their 3-round robin matches, and finished 25 points ahead of Mahut and Herbert for the No.

Mahut and doubles partner Herbert became the third all-French pair in the open-era to claim the French Open title as they defeated Oliver Marach and Mate Pavić 6–2, 7–6(7–4) in the final.

[22] Partnering with Édouard Roger-Vasselin Mahut reached the Wimbledon doubles final falling to Colombian duo Juan Sebastián Cabal and Robert Farah in five tight sets 7–6(7–5), 6–7(5–7), 6–7(6–8), 7–6(7–5), 3–6.

[24] Mahut reunited with Pierre-Hugues Herbert to play the Paris Rolex Masters which they won defeating Karen Khachanov and Andrey Rublev 6–4, 6–1 in the final.

Mahut continued partnering with Pierre-Hugues Herbert to play at the 2021 French Open where the pair won it for the second time in their career beating Alexander Bublik and Andrey Golubev in the final.

The home favorites saved three match points en route to the final defeating second seeds Juan Sebastián Cabal/Robert Farah.

[33] After an unsatisfactory American hard court swing, Mahut played at the 2021 US Open with Pierre-Hugues Herbert, they reached the quarterfinals, after three wins.

[37] At the 2022 Open Sud de France in February, he won his 40th title with partner Herbert as top seeds, defeating Lloyd Glasspool and Harri Heliövaara.

He has an accurate first serve that puts his opponent out of position, and then comes to the net to finish the point, a tactic he often employs on grass.

His consistent serve, skillful net play and clever placement of returns have also made him an exceptional doubles player.

In what became a record-setting match, spanning three days, qualifier Mahut faced 23rd seed John Isner in the first round of the 2010 Wimbledon Championships on 22–24 June.