Following a great beginning in his career with French clubs Caen and Marseille, the Clairefontaine graduate was bought by Chelsea in 2001. Claudio Ranieri bought the French centre-back from Marseille in 2001 for £6.2 million and developed partnerships with compatriot Marcel Desailly and later with future captain John Terry.
During his time at Stamford Bridge, Gallas obtained his first international cap for France in 2002. He was included in France's squad for the successful 2003 FIFA Confederations Cup on home soil and played the full 90 minutes as Les Bleus defeated Cameroon 1–0 in the final. He went on to represent France at the European Championships the following year in Portugal, starting in three of France's four matches, as they were knocked out by eventual champions Greece.
Meanwhile, back in London under new manager Jose Mourinho, Gallas' partnership with Terry saw them win back-to-back Premier League titles and a League Cup in 2005. While everything was fine in West London for the Frenchman, Gallas' contract with Chelsea was due to expire in May 2007. He refused to sign a new contract with Chelsea on improved terms after saying the club did not offer him enough money. Chelsea, however did not agree to a transfer request by the player in May 2006, as they considered Gallas an important first team member. Gallas refused to turn up at Chelsea's crowded ranks of defenders at Stamford Bridge, so Chelsea signed Dutch international centre-back Khalid Boulahrouz.
On September 1st, 2006, Gallas transferred to Arsenal as part of a deal that brought Ashley Cole to Chelsea with Arsenal also recieving £5 million. After the transfer, the club issued a statement that Gallas threatened to deliberately score own goals if he was not allowed to leave Chelsea. Gallas rejected Chelsea's claim and accused the West Londoners of "lacking class" and "hiding behind false accusations.'' In his five years at Chelsea, he played 159 matches and scored 13 goals. Gallas signed a four-year contract with Arsenal, where he took the number 10 shirt vacated by the recently retired Dennis Bergkamp.
At Arsenal within one year at the club, he was confirmed as Arsenal captain, with Kolo Touré serving as vice-captain. During his time, he scored against his former club in a 1-0 win. Gallas also attracted some criticism as a result of his actions in a 2–2 draw against Birmingham City on February 23th, 2008, a game in which his teammate Eduardo had his leg broken following a challenge by Martin Taylor. When Gaël Clichy conceded a penalty in injury time at the end of the second half, Gallas walked into the Birmingham half, apparently in protest. He stood there as Birmingham striker James McFadden converted the penalty. He then had to be restrained as he appeared to confront the crowd in fury. In November, Gallas gave an interview to the Associated Press in which he revealed tensions within the squad which were disruptive to team morale, and suggested that Arsenal's younger players needed to show more courage if they were to be successful, in which later he was stripped of his captaincy by Cesc Fabregas. At Arsenal, he played 101 matches and scored 12 goals.
Internationally, in the next FIFA World Cup, they were runner-ups in Germany over champions Italy and when the tournament in South Africa came along, they were considered one of the favourites for the title. As a senior member of the team, Gallas was cited as being one of the disruptive influences in an bitter camp in which after the tournament, Gallas criticised Domenech, blaming him for Les Bleus' first round elimination.
After the World Cup, he went on to play for another London club and bitter rivals of Arsenal, Tottenham Hotspur. While his first season went well, the second one was filled with several injuries which hindered his playing time. In two years he played 61 matches and scored only one goal. He finished his career at Perth Glory in Australia, where he retired from professional football after one season in 2014. He was also able to get 84 caps for his nation France and scored five goals, including one from a Thierry Henry handball against Ireland in a World Cup qualifier to allow French qualification to the 2010 FIFA World Cup.