A graduate of the French football academy at Clairefontaine, Gallas started his professional career at second division Caen, helping them to promotion as champions in 1996. He subsequently transferred to Marseille in 1997, where he played for four seasons. He made his debut in the UEFA Champions League for Marseille, scoring his first goal in European competition in a 1–0 win over then European champions Manchester United at the Stade Vélodrome in October 1999.

Gallas was a part of the France U-18 European Championship winning side. He also played for the France U-20 team in the 1997 FIFA World Youth Championship along with Thierry Henry, David Trezeguet, Nicolas Anelka, Mikaël Silvestre and Willy Sagnol.

Chelsea manager Claudio Ranieri bought Gallas from Marseille in 2001 for £6.2 million. He chose the number 13 shirt because it was his postcode back where he lived in Marseille. Under Ranieri, Gallas developed centre-back partnerships with Marcel Desailly and later with John Terry. His partnership with Terry included a run of 16 games without conceding a goal. He also played as a left-back at times. It was with Chelsea and under Ranieri where Gallas obtained his first international cap for France. Gallas made his debut as a substitute for John Terry against Newcastle United in August 2001. His first goal followed in the FA Cup against Tottenham Hotspur. Chelsea ended up reaching the 2002 FA Cup Final in which Gallas started, however they were defeated by Arsenal.

In October 2002, Gallas debuted for the France senior team in a UEFA Euro 2004 qualifier against Slovenia. He was included in France’s squad for the 2003 FIFA Confederations Cup and played the full 90 minutes as Les Bleus defeated Cameroon 1–0 in the final. He went on to represent France at UEFA Euro 2004, starting in three of France’s four matches, as they were knocked out by eventual champions Greece. In August 2005, Gallas scored his first goal for the national team in a friendly match against the Ivory Coast.

Due to a serious injury to left-back Wayne Bridge in 2004–05, Gallas was forced to play out of position, and despite the purchase of Asier del Horno from Athletic Bilbao for £8 million in the 2005 close season, Gallas found himself often playing on the left, a situation he grew increasingly frustrated with, although it did showcase his versatility, being able to play anywhere in the backline. Gallas scored some vital goals against Liverpool, Tottenham Hotspur and Manchester United as Chelsea retained their title in the 2005–06 Premier League season.

On 1 September 2006, Gallas transferred to Arsenal as part of a deal that brought Ashley Cole to Chelsea, in addition to Arsenal receiving £5 million. After the transfer, Chelsea 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 “lack[ing] class” and “hiding behind false accusations.” Claudio Ranieri, the former Chelsea manager who signed Gallas from Marseille, said of the allegations, “I cannot believe these stories. It is absolutely incredible. I signed William and he was always a good professional on the pitch. Of course, I don’t know what has happened in the last two years.” PFA Chief Executive Gordon Taylor declared that the whole episode has left a “bad taste in the mouth” and that Chelsea’s statement should be taken “with a large pinch of salt.”

Gallas made his full debut for the Gunners at left-back in the team’s 1–1 home draw against Middlesbrough on 9 September 2006 and scored his first goal against Sheffield United on 23 September 2006.

Gallas was ever present as France finished runner-up in the 2006 FIFA World Cup, losing on penalties to Italy at the Berlin Olympiastadion. In September 2006, he won his 50th cap for France in a 3–1 win against Italy at the Stade de France in a UEFA Euro 2008 qualifier. At UEFA Euro 2008, Gallas was a regular starter as France were knocked out at the group stage.

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 and expressed a wish to play in the Italian Serie A, with Juventus and Milan both interested. Chelsea, however, did not agree to a transfer request by the player in May 2006, as they considered Gallas an important first-team member. After the 2006 FIFA World Cup, Gallas refused to turn up at Chelsea’s crowded ranks of defenders at Stamford Bridge, so Chelsea signed Dutch international centre back Khalid Boulahrouz, though in his single season at Chelsea, Boulahrouz would play almost exclusively as a fullback.

Gallas was confirmed as Arsenal captain on 9 August 2007, with Kolo Touré serving as vice-captain. This caused some controversy, however, as Gilberto Silva was expected to receive the captain’s armband. His first competitive game as Arsenal captain was on 12 August 2007 against Fulham, which ended in a 2–1 victory for Arsenal. Gallas suffered a groin injury in the game against Blackburn Rovers on 19 August, but returned to play for Arsenal in their 2–0 victory over Bolton Wanderers on 20 October. In the traditional Arsenal–Manchester United clash at the Emirates Stadium on 3 November, he scored an own goal to initially give United the lead but also a last-minute goal to give the Gunners a draw.

On 24 November 2007, Gallas scored against Wigan Athletic when Bacary Sagna crossed in and Gallas headed past goalkeeper Mike Pollitt. On 16 December 2007, Gallas scored against his old club Chelsea, which was to be the only goal in the match to secure three points for Arsenal. He later reflected on his goal against former club Chelsea, claiming that, “When Cesc [Fàbregas] sent the corner in for my goal, I saw Petr Čech come off his line and to be honest I was sure he would catch it. So I was surprised when he didn’t take it, and it was just a reflex reaction to head it into the goal. I just happened to be there at the right time.”

Gallas attracted some criticism as a result of his actions in a 2–2 draw against Birmingham City on 23 February 2008, a game in which his teammate Eduardo had his leg broken following a challenge by Martin Taylor. When Gaël Clichy of Arsenal 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. After the final whistle, Gallas sat down on the pitch as the rest of the players left the field before being consoled by manager Arsène Wenger.

Gallas scored for Arsenal against Bolton at Reebok Stadium in a 2–3 win on 29 March 2008, starting the comeback which ten-man Arsenal ultimately completed. Amid speculation that Gallas would be replaced as club captain following his behaviour during the Birmingham game, Wenger said that he would consider the matter at the end of the season.

In August 2008, it was announced that Gallas would remain the club’s captain for the 2008–09 season. In the 2008–09 season, Gallas proved prolific in the Champions League. He scored six goals in Arsenal’s campaign, including two against Dutch side Twente and an equaliser against Dynamo Kyiv, as well as a header against Spurs. On 29 October 2008, he scored his first league goal of the season against Tottenham at the Emirates, scoring Arsenal’s second goal in a 4–4 draw. On 8 November 2008, in a 2–1 victory over Manchester United, he received praise for his display whilst playing alongside former United defender Mikaël Silvestre. It was claimed he outshone United defenders Rio Ferdinand and Nemanja Vidić.

On 17 March 2009, Gallas scored the winner in a 2–1 win against Hull City in the quarter-finals of the FA Cup. On 10 April 2009, it was confirmed that Gallas would miss the remainder of the season after damaging the medial ligament in his right knee during the 2008–09 UEFA Champions League quarter-final first leg tie against Villarreal. The match finished 1–1 with Gallas only lasting 43 minutes of the first half.

In November 2009, Gallas scored a goal which qualified France for the 2010 FIFA World Cup. The goal came against the Republic of Ireland in extra time of a World Cup playoff, making the score 1–1, and 2–1 to France on aggregate. The goal was controversial as it was scored after his teammate, Thierry Henry handled the ball twice.

On 20 August 2010, Tottenham Hotspur manager Harry Redknapp confirmed that Gallas was set to join the club and described the move as a “no-brainer.” The transfer was confirmed on 22 August, with Gallas signing a one-year contract at White Hart Lane. According to Tottenham’s club historian, the move meant that Gallas would become the first player to have appeared for Chelsea, Arsenal and Tottenham; Clive Allen played for Tottenham and Chelsea and had a spell with Arsenal early in his career, but did not make an appearance for them. After training with his new team for the first time, Gallas stated that he believed that Spurs have the potential to win the Premier League.

Gallas made his first-team debut on 11 September in a 1–1 draw away at West Bromwich Albion. On 20 November, as a result of injuries to regular captains Ledley King and Michael Dawson, Gallas captained Tottenham for the first time against his former club Arsenal at the Emirates Stadium in a famous 2–3 Tottenham victory, Spurs’ first league victory at Arsenal since 1993. On 2 February 2011, Gallas partnered Sébastien Bassong at the back against Blackburn at Ewood Park in their 1–0 midweek victory. Gallas continued to impress for Tottenham into 2011. In the Champions League knockout stages, he put in two commanding performances against Milan to set up a quarter-final tie against Real Madrid in the club’s first attempt in the competition.

Despite a season of frequent changes in defence for Tottenham due to a host of injuries to his fellow defenders, Gallas performed strongly throughout his first season with the club and was rewarded with a two-year contract extension in March 2011, which kept him at the club until the end of the 2012–13 season. In his debut season at Spurs, he made 36 appearances in all competitions.

On 6 June 2011, aged 33 and with 84 caps to his name and 5 goals he announced his retirement from international football with immediate effect after he had not been selected in the year since the World Cup. Gallas was eligible to represent Guadeloupe internationally as well, as they are not FIFA-affiliated. His cousin Ludovic Quistin has represented Guadeloupe.

During the 2011–12 Premier League, Gallas suffered several injuries that hindered his playing time for the season, making just 19 appearances for Tottenham during the campaign, particularly in light of the competition for the center-back spot with Michael Dawson and Ledley King. In the following season, Gallas played 17 games in the Premier League, scoring his first goal for the club—and also his first goal in over two years—against former club Chelsea in a 4–2 defeat. On 7 June 2013, the Premier League confirmed that he had been released from Tottenham.

On 23 October 2013, Gallas signed a one-year deal with Perth Glory, becoming the first Frenchman to play in the Australian A-League. Gallas joined the league following the arrival of other international players, Alessandro Del Piero, Emile Heskey and Shinji Ono. Gallas made his debut for Perth in round 6 against Adelaide United, coming on as substitute. He scored his first goal on 3 September 2014 in a 1–1 draw against Wellington Phoenix He retired after the end of the 2013–14 A-League, making 14 appearances and scoring one goal.