A product of the Clairfontaine academy, William started his career with Caen in France, where he was part of the side promoted to Ligue 1, then stepping up to play for Marseille at the top of the table and in the Champions League, Gallas earned himself a reputation as a pacy defender, who read the game well and had a steely determination to win.
This was spotted by Chelsea manager Claudio Ranieri, who brought the defender to the Premier League and he settled in well at Stamford Bridge, where he played in the middle of defence with Marcel Desailly or John Terry or sometimes he was played at right back. His form earned him a first French cap.
With a change in management, Jose Mourinho arrived and took the club to two Premier League titles and a League Cup win, but injuries had forced Gallas to be deployed at left back more often and he was not happy with this and the fact that he had not been offered enough money in a new deal. With his contract due to expire in May 2007, so, with a year left on his contract, he asked to leave and his transfer request was refused.
After the 2006 World Cup finals, Gallas refused to turn up for training, as Chelsea had bolstered their defence during the summer, leaving the club no option, but to sell him in September 2006 to Arsenal in a deal that took him and £5 million to Highbury and Ashley Cole to Chelsea. The West London club revealed that the defender had threatened to score own goals if they did not let him leave.
His four year deal at Arsenal started well and in August 2007 he took the captain's armband and was sometimes a controversial choice to lead the side, having been disappointed with a 1-1 draw and having seen his team-mate Eduardo suffer a broken leg at Birmingham City in February 2008, he berated his fellow defenders for the last minute goal they conceded and sat on the pitch on his own in the centre-circle after the final whistle. Another moment of criticism arose in November 2008, when he spoke out about the younger members of the squad needing to show more courage to be successful, which resulted in him being dropped for one match, fined two weeks wages and being stripped of the captaincy.
An injury to his knee ligaments towards the end of the 2008-09 season saw him miss a month's football and then another injury kept him out for two months, returning against Barcelona in the Champions League, only to be stretchered off before half-time in what proved to be his last Arsenal appearance.
With a disagreement over how long a contract he should be offered by Arsenal at the end of the 2009-2010 season, he became a free agent, when the club did not make a suitable offer and he was signed by Spurs on a free transfer. With injury worries over experienced central defenders Ledley King and Jonathan Woodgate, Harry Redknapp wanted an old head to play alongside Michael Dawson or Sebastien Bassong. With Woodgate only playing less than a half a game and King only eight matches, his signing was vital as Tottenham faced their first Champions League campaign.
Gallas' ability to read the play and nip in ahead of strikers helped him, as he was clearly hampered all season with hamstring and groin problems, but he performed at a high level and rose to the occasion in the Champions League games, showing he was still a fine competitor and proved that as Redknapp said when he signed him, it was "a no brainer".
His initial one year contract was extended to the end of the 2012-13 season on the back of his impressive performances during his first season with the club and Gallas put in some solid performances, reading the game well to counter his lack of pace. However, he began to suffer with calf muscle problems and these carried on the following season when the option of a further year was taken up. William started to make some mistakes, which cost goals and while he also played some decent matches in the absence of Younes Kaboul for most of the campaign, but Vertonghen, Dawson and Caulker were becoming the preferred options and he made no more appearances after the home tie against Basle in the Europa League.
The defender was released by Spurs on 7th July 2013.