Rushden & Diamonds Williamson joined
Rushden & Diamonds Centre of Excellence at the age of 6. On that period, Williamson said, "My coach at the time left to go to Arsenal. I went across with her and had a couple of trials and luckily they took me on and I never looked back from there. So I owe it all to my coach from when I was 9."
Arsenal Williamson joined
Arsenal's
Centre of Excellence in 2006 at the age of nine. She considered quitting football at the age of fifteen, skeptical about the possibility of playing professionally. She made her
FA WSL debut on 16 April against
Notts County. 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. On 13 July, Williamson netted her first professional goal for Arsenal in a 4–0 win away against
Millwall Lionesses in the
League Cup. On 4 September, she scored her first league goal against
Chelsea. She made 12 appearances in the
2014 FA Women's Super League. Williamson played in the
2014 League Cup final against
Manchester City which Arsenal lost 1–0. 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.
2015 On 8 January, Williamson won the
England Women's Youth Player of the Year award. Before the start of the
2015 FA WSL, on 31 March, Williamson signed her first professional contract with Arsenal. On 26 April, she was named the
2014–15 PFA Young Women's Player of the Year. Speaking about Williamson following the award, then-Arsenal head coach
Pedro Martinez Losa said “I’ve worked with some of the top players in the world like Kelly Smith and
Abby Wambach and when you work with them day by day, you realise what this player can do. Leah is made for big challenges and big things.” Williamson scored her first goal of the season in a convincing 3–0 home victory over
Watford in the
League Cup on 23 July. Williamson was a key figure in the club's victorious League Cup campaign, after the quarter final win over holders
Manchester City, who had beaten Arsenal in the final, Williamson said, "I think Arsenal as a football club have won a lot of trophies in the past and we wanted to keep winning silverware for the club. Even with the younger players in the squad we all thought we have got to win, the morale was certainly high before the game. Last year’s final was always in the back of our mind and we know how good a side City can be. After going 1–0 up it was all about defending which we managed to do and it was a great result for us. I think we can win the cup as we have won it three times but we have to keep on focusing on the next game." Having appeared in 13 out of the club's 14 WSL matches in the 2015 season, on 31 July 2015, it was announced that Williamson had signed a new long-term contract with Arsenal.
2016 Williamson helped her team reach the
2016 FA Women's Cup final, which Arsenal won by a 1–0 margin over
Chelsea. On 16 June 2016, Arsenal announced that she had successfully undergone surgery on her ankle.
2017–18 Williamson was ever-present in the
2017–18 Women's FA Cup as Arsenal defeated the defending champions,
Manchester City in the
final. Arsenal's league campaign was more disappointing with Arsenal missing out a top two finish and
Champions League qualification for the sixth consecutive edition of the WSL. She started the season in midfield but switched to central defence with the arrival of new manager
Joe Montemurro in December 2017. On her change of position, Williamson said, "I'm not sure in the long term and I never want to shut the door on either position, because I know I can do both. But I'm enjoying playing as a centre back with the way we play. Under Joe we want to play out from the back and that suits my qualities. At the minute I am enjoying playing at centre half, but I'm open minded about what the future holds." Towards the end of the season, Williamson reflected on the season since Montemurro took over, "I feel like since Joe came in we're back on a journey again, we had lost that a little bit. Despite today, I feel like we're building something. But pretty much every game we have played since around November has been a must win game and that takes its toll emotionally. In every single game we have known no less than a win will do, in the league and in the cups."
2018–19 On 14 October 2018, Arsenal laid down a marker in their title challenge with a 5–0 thrashing of champions
Chelsea, inflicting a first home defeat on the champions since July 2016.
Joe Montemurro began utilizing Williamson on the right of a back three and after a 4–0 win at
Bristol City on 28 October, in which Williamson assisted
Jordan Nobbs for one of the goals, he commented positively on her performance. 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. Two games later, on Williamson's 100th Arsenal appearance, a 2–1 defeat to Chelsea allowed Manchester City to move a point clear albeit having played one game more. On 23 February, Arsenal attempted to defend their title in the
2019 FA Women's League Cup final, but Manchester City won 4–2 on penalties; nevertheless, Montemurro praised Williamson after the match. 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. The team won another four games in a row to clinch the title with a 4–0 defeat of
Brighton at the
Falmer Stadium on 28 April. During the season, the most common passes exchanged were between Williamson and
Lisa Evans at right wing back. Williamson took the most touches out of any Arsenal player, with 1,501, and completed the most passes in the opposition’s half (591). She also made thirty interceptions, more than any other player in the squad.
2019–20 Williamson was voted to the PFA Team of the Year for 2019–20. Due to the
COVID-19 pandemic, the season was cut short and
Chelsea were announced the eventual winners.
2020–21 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. On 18 November, Williamson made her 150th appearance in all competitions for Arsenal against
Tottenham Hotspur in the
League Cup, a game which Arsenal would draw 2–2 but go on to win 5–4 on penalties.
2021–22 On 16 June 2021, Arsenal announced that Williamson had signed a new one-year contract with the club until the end of the 2021–22 season, with Williamson saying the decision to stay following the departure of manager
Joe Montemurro had been tougher than before. On November 13, she suffered a signfificant hamstring injury in the match against
Tottenham Hotspur. Teammate
Lotte Wubben-Moy dedicated her goal in the next game to Williamson. She returned to play after two months. Just seven months after signing the one-year contract, in January 2022, she agreed to a prolonged contract with the club. Arsenal did not disclose the duration of the new commitment, but coach
Jonas Eidevall stated that Williamson is "a player for us to build Arsenal around".
2022–23 On 21 April, she announced her injury from two days prior was a ruptured ACL, ruling her out of the
Champions League and the
Women's World Cup later that year.
2023–24 Williamson returned from injury on 21 January 2024 to play in the
League 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. In May 2024, Williamson signed a new contract with Arsenal. The duration of the contract was not disclosed. Speaking about the new contract, Williamson said "“Everyone knows what Arsenal means to me, but I think every time I sign a new contract, I feel that love ignite all over again. I’m very happy to be staying and looking forward to the next couple of years."
2024–25 Williamson was ruled out of both the
opening WSL game against
Manchester City and
Champions League game against
BK Häcken, due to concussion protocol. Following a tough start to the season, Williamson faced heavy criticism on social media about her performance. On 14 March, during Arsenal's match against
Everton, Williamson broke the record for the number of appearances for her club in the WSL. She had made 158 appearances, 146 of which were starts for the club. On May 24, she won the UEFA Women's Champions League with Arsenal, in a 1–0 win against
Barcelona.
2025–26 On 21 August 2025, it was announced Williamson would miss the start of the 2025–26 season, after undergoing knee surgery to cleanup an injury sustained in the Euros final. She returned to the pitch on 13 December 2025, coming on as an 82nd minute substitute in a 3-1 victory over
Everton. On 30 April 2026, Arsenal announced that Williamson had signed a new contract with the club, which the BBC reported as a two-year deal. ==International career==