He began his career at minor club ES Arques-la-Bataille before being signed by Arsène Wenger's Monaco side at the age of 18. He made his debut soon after and played in the 1989 Coupe de France final. Petit became a regular at Monaco, playing as a left-sided or central midfielder. In 1991, he won the Coupe de France with Monaco and also played in the 1992 European Cup Winners' Cup final (which Monaco lost to Werder Bremen). In 1996–97, his final season at Monaco, he captained his side to the Ligue 1 title.
Despite interest from Levski Sofia, Rangers and Tottenham Hotspur, Petit joined Arsenal in June 1997 for £2.5 million, where he was reunited with his former Monaco manager Arsène Wenger. Wenger switched Petit from central midfielder to defensive midfielder, and partnered him with fellow Frenchman Patrick Vieira. The French duo formed a midfield partnership which brought instant success, as Petit won the double with Arsenal in his very first season, clinching both the Premier League title and the FA Cup. Altogether, in the three seasons in his Arsenal career, Petit made 118 appearances and scored 11 goals, including a stunning drive from outside the area against Derby County (which was also the winning goal), during the 1997–98 season.
Petit moved to Barcelona (together with Arsenal teammate Marc Overmars) in the summer of 2000 for £7 million (€14 million). At Barcelona, he was moved back into defence and suffered a rash of niggling injuries. As a result, he failed to settle and could not hold down a regular place. In his biography, published in 2008, the midfielder gave his time at Barcelona a special chapter in which he exposed that coach Lorenzo Serra Ferrer did not even know what position he played when he joined the team. His only goal for Barcelona came on 13 May 2001 at home to Rayo Vallecano in a 5–1 win.
After his first season at the Camp Nou, Petit was linked with moves back to England with Manchester United, Tottenham and Chelsea, joining the latter in 2001 in a £7.5 million transfer deal. He initially was a first-team regular for the Blues in a largely disappointing first season and played in the 2002 FA Cup final which Chelsea lost to his old club, Arsenal. He scored his first goal for Chelsea in a 2–1 win over Derby on 30 March 2002. His second season saw a significant improvement, as he formed an impressive midfield partnership with Frank Lampard as Chelsea impressed in the winner-takes-all final game of the season against Liverpool as Chelsea secured the fourth UEFA Champions League berth. He also scored twice throughout the season: against Everton in the League Cup, and former club Arsenal in the league. After a series of knee injuries, however, he spent much of his final season of his career on the sidelines, and he was released on a free transfer in the summer of 2004, his final appearance for the club coming against Blackburn Rovers on 1 February 2004.
After being released by Chelsea, Petit rejected the chance to sign for Bolton Wanderers, and he announced his retirement on 20 January 2005 after failing to fully recover following knee surgery.
Playing for the France national team, Petit earned 63 caps and scored six international goals in his career and won the 1998 FIFA World Cup and UEFA Euro 2000. He scored twice in the 1998 World Cup, the first from a powerful shot from just outside the box against Denmark, which turned out to be the match winner, and a second in the final against Brazil. The goal he scored in the final was particularly memorable, as he had embarked on an optimistic run across field before calmly slotting in the goal in the final minute of regular time. That same goal happened to be the 1,000th goal in the history of the French Football Federation, and the last World goal of the 20th century. France won the match 3–0. An earlier Petit corner kick had set up Zinedine Zidane's header for France's first goal. Petit was also part of the 2002 World Cup squad, though France failed to advance past the group stages and failed to score a single goal in three matches during their defence of the trophy.
Petit retired from international football in September 2003.
Petit married French actress Agathe de La Fontaine, in 2000, but later divorced her in 2002 after having one child, Zoe. He now shares a life with Maria Servello, with whom he had another child, Violet, in 2007. He has often appeared on French TV as a football analyst.
His brother Olivier was an amateur footballer when Petit was a teenager. While playing for his club Arques in 1988, Olivier collapsed and was rushed to hospital where he was pronounced dead due to a blood clot of the brain. In his biography, Petit explained that this event shocked him to such an extent that it almost made him quit football. He viewed his brother as a gifted young man who had good looks and did well in all his endeavours, whether it was football or education.
In July 1998, Petit won £17,000 worth of francs on a fruit machine in a Monte Carlo hotel. He gave it all to charity.
Petit's squad number at all his clubs was 17.
In June 2011, Petit shaved his hair for charity, raising £34,000, he also shaved his beard. Petit has been an ambassador for the Homeless World Cup movement since the tournament was hosted by Paris in 2011. He also featured in an episode of The Bill in a Christmas special in 1998.
In May 2016, Petit signed on with online trading broker UFX.com to serve as the company's French brand ambassador. He appears in UFX's Trade with the Pros online and television campaigns, alongside British brand ambassador and former rugby player, Mike Tindall, MBE.Monaco
Ligue 1 (1): 1996–97
Coupe de France (1): 1990-91
FA Premier League (1): 1997–98
FA Cup (1): 1997–98
FA Charity Shield (2): 1998, 1999
FIFA World Cup (1): 1998
UEFA European Football Championship (1): 2000
French Division 1 Rookie of the Year (1): 1990
Premier League Player of the Month (1): April 1998
Onze de Bronze (1): 1998
PFA Premier League Team of the Year (1): 1998-99
Légion d'honneur: 1998