Mathieu was born in Strasbourg, France. He first began playing tennis at age 3, with his older brother Pierre-Yves. Between 1997 and 2000 he trained at the Nick Bollettieri Tennis Academy in Bradenton, Florida.
As a junior Mathieu posted a singles record of 42–15 (and 34–12 in doubles), reaching as high as No. 6 in the world in January 2000 (and No. 19 in doubles the same month).
Mathieu won the boys' singles title at the 2000 French Open, (defeating Tommy Robredo in the final) and made his ATP tour debut that July in Kitzbühel.
2002 was Mathieu's breakthrough year. He made the fourth round of the French Open, losing to Andre Agassi in five sets, despite having a two-set lead. Later on in the year, he confirmed his potential by winning back-to-back tournaments in Moscow and Lyon. He holds the distinction of being the last player to beat Pete Sampras before his retirement, which he did at the 2002 TD Waterhouse Cup. On 14 October, he became world no. 36, and his progress won him the ATP Newcomer of the Year award for 2002. He also nearly won the Davis Cup in 2002 with the French Davis Cup team, but lost the deciding rubber of the final to Mikhail Youzhny of Russia, once more after relinquishing a two-set advantage.
In 2005, he achieved his best result in an ATP Masters Series event, knocking out Andy Roddick on his way to the semifinals at Montreal. He had a record of 2–2 in the four Davis Cup matches he played that year. He won both his matches against the Swedish opponents Thomas Johansson and Joachim Johansson, but lost to Russia's Nikolay Davydenko and Igor Andreev in the quarterfinal tie.
2006 saw him equal his best result at a Grand Slam tournament by reaching the fourth round of the Australian Open. In May a career-high ranking of no. 32 was attained. In the third round of the French Open, he lost to eventual champion Rafael Nadal in a gruelling encounter which lasted 4 hours and 53 minutes, but amazingly only saw 42 games played (Nadal won the match 5–7, 6–4, 6–4, 6–4, with the first set lasting 93 minutes and each of the following sets longer than an hour). Many tennis players and commentators, including two-time French Open runner-up Àlex Corretja, hailed it as a classic.
2007 started poorly for Mathieu when he injured himself at the Australian Open during a 1st round encounter against Spaniard Fernando Verdasco and was forced to retire from the match. This was unfortunate as Mathieu was up 2 sets and 3–0 in the 3rd set tiebreak at the time. After returning from his injury, he reached the 4th round in Miami, beating then world number 5 Fernando González of Chile along the way, before bowing out to Andy Murray in 3 sets.
On 29 April 2007, Mathieu won his 3rd career title, the Grand Prix Hassan II in Casablanca defeating Álbert Montañés 6–1, 6–1. At Wimbledon, he reached round 4 for the first time, defeating Radek Štěpánek, No. 17 seed (15th-ranked) David Ferrer, and 15th seed (12th-ranked) Ivan Ljubičić. He attained a career high ranking of 28 in singles after this result, entering the world's top 30 for the first time. The week after Wimbledon, he beat Italian Andreas Seppi 6–7, 6–3, 7–5 in a difficult final to claim his fourth ATP Tour title in Gstaad, Switzerland. He rose to No. 23 in the rankings, making his top 25 breakthrough.
At the Montreal Masters, he produced one of the comebacks of the season to beat 15th seed Guillermo Cañas. Trailing 4–6, 0–4, he managed to up his level of play to win 13 of the next 14 games and record a win by the score of 4–6, 7–5, 6–0. He followed that up with a win over Mario Ančić in round 2. In round 3, he ran into Rafael Nadal, and actually won the first set 6–3 before losing the next two 6–3, 6–2.
He then made the semi-finals of New Haven losing to world number 6 James Blake in a 3rd set tiebreak. This result projected him in the world's top 20 for the 1st time, at the 20th rank.
At the 2012 French Open, Mathieu won his first round match from two sets down before defeating John Isner in five sets, 18–16 in the decider in what proved to be the second longest match in French Open history and fourth longest in Grand Slam history. He lost in the third round to the Spaniard Marcel Granollers. Mathieu defeated Igor Andreev of Russia in the Swiss Open [6–3, 7–6 (4)].
At the 2015 Generali Open Kitzbühel, Mathieu reached the final as a qualifier, after wins over Kenny de Schepper, Martin Kližan, Federico Delbonis and Nicolás Almagro. He lost in the final to Philipp Kohlschreiber 2–6, 6–2, 6–2.
Mathieu is known for his exceptionally clean groundstrokes on both wings, with his heavy topspin forehand probably being his biggest weapon.
A relatively popular, well-liked player despite his inconsistent career results, Mathieu is often affectionately known by his initials, PHM. His favourite surfaces are clay and hard, and he admired Boris Becker while growing up. His brother Pierre-Yves is now a tennis coach in Strasbourg. In March 2012, Mathieu became a father for the first time when his wife, Quiterie Camus, gave birth to the couple's first child, a son named Gabriel.
Current till 2017 Wimbledon Championships.