On 12 November 2011 Williams made his debut in the FA Cup victory over Nantwich Town, briefly becoming Milton Keynes Dons' youngest player, before scoring his first professional goal making him Milton Keynes Dons youngest goalscorer being 16 years, two months and five days old. The goal also made him the youngest scorer within the FA Cup proper rounds.
On 14 June 2012, Williams joined Premier League side Fulham as a first year academy scholar for the 2012–13 season after an undisclosed compensation package was agreed between the two clubs. Following his move to Fulham, MK Dons chairman Pete Winkelman said he was disappointed seeing Williams leave the club, and told the club's official website: "We are very disappointed that George has chosen not to take up his scholarship or the offer of a professional contract with MK Dons and has instead decided his future is best served with Fulham".
Loan spells
On 16 February 2015, Williams rejoined League One side Milton Keynes Dons on a youth loan for the remainder of the 2014–15 season, but his loan was cut short by an anterior cruciate ligament injury, keeping him out for between four and six months. On 12 February 2016, Williams joined League One side Gillingham on loan until the end of the 2015–16 season. On 29 July 2016, Williams returned to Milton Keynes Dons on loan until the end of the 2016–17 season. On 31 January 2018, Williams joined Scottish Premiership side St. Johnstone on loan until the end of the 2017–18 season.
Williams qualifies for Wales through his mother and represented Wales at under-17 and under-19 levels. On 5 March 2014, Williams made his Wales under-21 debut as an 83rd-minute substitute in a 1–0 defeat to England. On 27 May 2014, he was called up to the senior Wales squad for a friendly match against Holland to be played on 4 June 2014 as a replacement for the injured Gareth Bale. He made his international debut as a replacement for Jonathan Williams in the 70th minute of the match. Williams was included by manager Chris Coleman as a member of Wales' Euro 2016 squad that reached the semi-finals of the tournament, although he did not make an appearance.
