[12] In 2006, she was selected as the mascot to the Arsenal men's team in a league cup match at the Hawthorns against West Bromwich Albion.

[17] Williamson made her senior team debut the day after her seventeenth birthday when she came on as an 81st-minute substitution for Rachel Yankey in Arsenal's 0–2 defeat to Birmingham City in the UEFA Women's Champions League quarter-final.

[19] She won her first major title, the 2014 FA Women's Cup with Arsenal on 1 June in the final against Everton, in which she came on as a 76th-minute substitute for Jade Bailey.

[20][failed verification] On 13 July, Williamson netted her first professional goal for Arsenal in a 4–0 win away against Millwall Lionesses in the League Cup.

[23] At the end of the 2014 season, she was named the League Cup Player of the Year, ahead of Jess Clarke from Notts County, Ji So-yun from Chelsea and Toni Duggan from Manchester City.

[33] Arsenal won their first nine games and went into the match with Manchester City six points clear of their title rivals; however, they lost 2–0 amid an injury crisis.

[34] On 23 February, Arsenal attempted to defend their title in the FA WSL Continental Cup but Manchester City won 4–2 on penalties; nevertheless, Montemurro praised Williamson after the match.

[35] In the league, Arsenal won four games in a row, and after a 5–1 victory at Liverpool on 24 March, England manager Phil Neville indicated his confidence that Williamson would one day captain the national team.

[41][42] On 13 September 2020, Williamson netted her first goal of the season in Arsenal's 9–1 victory over fellow London side West Ham United.

[48] On 21 April, she announced her injury from two days prior was a ruptured ACL, ruling her out of the Champion’s League and the World Cup later that year.

[49] Williamson returned from injury on 21 January 2024 to play in the Conti Cup against Reading, where she came on as a substitute for Jen Beattie in the 61st minute and assisted a goal in the 6-0 win.

[61] On 9 April 2015, Williamson scored a penalty for her team in a qualifying match against Norway for the 2015 UEFA under-19 championship, which had been ordered to be retaken five days after the original contest.

The laws of the game state that in such a situation, the penalty should be retaken, but the German referee, Marija Kurtes instead awarded Norway a free kick.

[65] On 8 June 2018, she made her debut for the senior team coming on for Walsh with six minutes left in their 2019 FIFA World Cup qualifier against Russia, which England won 3–1.

[71] Then-manager Phil Neville felt he did not use Williamson as much as he could have during the qualifiers and World Cup, putting in more experienced defenders and considering Keira Walsh crucial to the defensive midfield instead.

[73] On 17 September 2021, Williamson was named England captain for the FIFA World Cup qualifier against North Macedonia at St Mary's Stadium, Southampton.

[75] Williamson captained the England squad which won the UEFA Women's Euro 2022;[76] she made the most ball recoveries in the competition[77] and was named to the Team of the Tournament.

[79][80] As European champions, England qualified to contest the first Women's Finalissima; on 6 April 2023, Williamson captained the Lionesses to another international title by defeating Brazil on penalties after 90 minutes.

The injury was later revealed to have been an anterior cruciate ligament, meaning she would miss the rest of the season and the 2023 World Cup in Australia & New Zealand.

She was proud of her defensive work in the team's group games, and of being an Olympian, saying she had to close her eyes when the camera passed her during the anthem as she was tearing up.

[85] Williamson has said that a shift in her mentality came when GB was knocked out of the Olympics, not wanting to feel the same disappointment, "she recognised what she needed to do to fulfil her potential" in terms of the demands of international competition.

[89] She also has by far the highest rate of passes into the final third for a defender in the WSL since 2021, with a November 2022 Arsenal technical report saying that "she wrecks the curve" for this statistic due to the margin.

[91] As part of the "Where Greatness Is Made" campaign, a plaque honouring Williamson was installed at Scot Sports & Social Club in Milton Keynes.

"[101] The day after England triumphed at the 2022 Euros, the team wrote an open letter to Tory leadership candidates Rishi Sunak and Liz Truss requesting government support for girls to receive two hours a week of PE lessons and to support resources for girls’ football sessions in school.

[104] In August 2023, Williamson travelled to Jordan and visited the largest Syrian refugee camp in the world in support of the Coaching for Life programme.

[105][106] Williamson was the first England women's footballer to address the United Nations when she appeared at the Sustainability Development Goals (SDGs) Summit in September 2023.

[108] Williamson has co-authored two books to date: You Have the Power: Find Your Strength and Believe You Can (2023)[109] - A positive guide for empowering young girls, written with sports journalist Suzanne Wrack.

The Wonder Team and the Forgotten Footballers (2023)[110] - A time-travel adventure for children aged 8–12, written with Williamson's cousin Jordan Glover; illustrations by Robin Boyden.

[115] Williamson has endometriosis, which made her anxious to play while menstruating, and has spoken about it to increase awareness of the disruptive condition and the need for more symptom-relieving measures.