Williamson joined Arsenal’s Centre of Excellence in 2006 at the age of nine.

Williamson grew up in Milton Keynes and lived close to the town’s stadium. She attended Ousedale School in Newport Pagnell for secondary education. She comes from an Arsenal family and has been a ‘Gooner’ since birth In 2006, she was selected as the mascot to the Arsenal men’s team in a league cup match at the Hawthorns against West Brom. She is training to be an accountant. Her best friend in football is international teammate and Man City midfielder Keira Walsh, they have gone through all the junior ranks together and received their first senior call up on the same day in 2017. Williamson said, “It’s the first thing you do, you check for your name on the list and then you check for Keira’s”. Walsh added, “Nobody I would rather have shared this journey with”.

Williamson made her senior team debut as 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. She continued to make 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 Man 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.

Williamson has represented England since 2010 at youth levels, currently playing in the under-23 squad. She captained the England under 17 side in the 2014 UEFA Women’s Under-17 Championship held in England in November and December 2013, where they eventually finished fourth after being beaten by Italy in the third place play-off, losing 3–4 in the penalty shootout. She was also part of the England women’s national under-20 football team to compete in the 2014 FIFA U-20 Women’s World Cup in Canada in August 2014. Williamson started in the starting lineup in every England’s match. The tournament, however, was a disappointment for the team as they were not able to progress through to the quarter finals, finishing third in their group with only two draws against South Korea and Mexico respectively, and a defeat to Nigeria.

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. In an unprecedented decision, UEFA had ordered the match to be restarted from the 96th minute.

Williamson was ever present in the 2017–18 FA WSL Cup as Arsenal ended as Champions beating the defending champions, Man City in the final. Arsenal’s league campaign was more disappointing with Arsenal missing out a top 2 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.”

In November 2017, Williamson was called up the senior England squad. She made her debut for the senior team with six minutes left in a 2019 FIFA Women’s World Cup qualifier against Russia in the team’s 3–1 win. Williamson was part of the squad who won a 2–0 victory over Denmark at Banks’s Stadium in Walsall for one of their final friendly preparation matches for the World Cup.

On 14 October 2018, Arsenal laid down a huge marker in their title challenge with a 5–0 thrashing of champions Chelsea, inflicting a first home defeat on the champions since July 2016. Williamson said, “This result means everything, regardless of league standings, we want to be the Arsenal of old, the Arsenal that everyone doesn’t want to play.” As a die hard Arsenal fan, the win meant a lot to Williamson. “I said to Mitch [Emma Mitchell] as we were walking over, as an Arsenal supporter I hope that they feel it, that I feel exactly what they’re feeling watching this game. First and foremost I’m a supporter of this football club.” Joe Montemurro began utilizing Williamson on the right of a back 3 and after a 4–0 win at Bristol City on 28 October, in which Williamson assisted Jordan Nobbs for one of the goals, he said, “Leah’s really adapting to a bit more of a pro-active defending methodology. She’s got a bit of freedom to go forward and I’m really enjoying working with her. She’s getting the rewards for the amazing work that she’s put in over pre-season and the season so far, so all credit to her.” Arsenal won their first 9 games and went into the match with Man City 6 points clear of their title rivals. By the time Arsenal faced Man City on 2 December, their injury crisis was such that they were forced to start 17 year old Paige Bailey Gayle in an injury hit team that lost 2–0 to a brace by Georgia Stanway. Two games later, a 2–1 home defeat to Chelsea which happened to be Williamson’s 100th Arsenal appearance allowed Man City to move a point clear albeit having played one game more. On the injury situation, Williamson said, “Obviously injuries are a factor and everyone can see we’re down to the bare bones at the moment and it was before Christmas. But genuinely, we are not a team that makes excuses and it’s not something we’re talking about. We are focusing more on how to get the ball from A to B, for example.” On the achievement of reaching 100 appearances at the age of just 21. Williamson added, “It’s funny, I was just talking to my mum and we said I could have done it much more quickly were it not for injuries. Naturally, I am always striving for more. In our meeting room at the training ground, the names of all the women who have made 100 appearances are on the wall and I look at some of the names on that wall and, without being too much of a fan girl, a lot of them are my heroes. It’s amazing to be up there now in their company. But it’s about the future for me too and the first 100 is amazing and especially at my age, I really do aim to get a few hundred more yet with this football club and in this shirt. This is more than just a football club to me. My family are all Arsenal supporters and they remind me every day that I am living a dream to play here and I hope that I have made them proud. Important day for me and them, I just wish we could have won”. On 23 February, Arsenal attempted to defend their title in the FA WSL Continental Cup but after 120 goaless minutes, Man City won 4–2 on penalties. On Williamson, Joe Montemurro said, “For a girl of her age to show the maturity and the ability to understand the game, the way she positions herself- her positioning is like a 30 year old pro. She always finds the pass, she’s unbelievable. She has all the attributes to be an amazing footballer and an amazing centre-back on the world scale and England should be very excited about the prospect of her going to a World Cup with them. We love having her at Arsenal.” On the cup final defeat, Williamson said. “That gives you a good idea of the character and resilience of this team. But also we’ve got players that know what it means to be Arsenal and we’ve got new players that already know what that means. Today we maybe didn’t play the Arsenal way, but that character is what it takes to play for a top team to take a game as far as you can, to fight and when it comes to the league, that’s exactly what we will do. We’re pissed off now and we’ve only got one target to focus on and we’ll give it every thing we’ve got.” Back in the league, Arsenal won 3 games in a row, scoring 10 goals while conceding none after the Chelsea defeat before an away tie with Liverpool on 24 March which Arsenal won 5–1 on a terrible pitch. England manager, Phil Neville was present and while watching Williamson ahead of the world cup said, “This girl is a star. You get some players who are born to captain England. She’s a leader. She’s only 22, and she comes into the squad and acts like a 30-year-old.” Arsenal 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. As a childhood Arsenal fan it meant the world to her and she said, “I came here when I was 9 and we were winning the league every year and it was all I ever wanted, to win the league with Arsenal. I guess I thought I would walk into the team one day and it would just happen. But the game has developed so much and the competition is so fierce, to win that out there is absolutely unbelievable. I thought it might come sooner, but the wait has made it sweeter.” On the injuries Arsenal suffered throughout the season, Williamson said, “That makes today even greater, because we’ve had a crazy season with injuries. It’s easy to forget that when the trophy is in the cabinet. It’s easy to forget that at one stage we were down to ten senior outfield players. Those two defeats [to City and Chelsea either side of Christmas] was during the readjustment period. But losing those two games was probably good for us in the long run because we knew there was no room for error, so we’ve played every game like a cup final. I do just wish we could have played that City game more towards the start of the season when we were on a great run playing unbelievable football. A lot of teams would crumble in those circumstances but we didn’t.” Williamson was typically modest in her own performance in the title clinching match, “I think that might be the worst I’ve ever played for the club and it’s because of how much this game and this title means to me. That’s something I’ll learn from, but I won’t apologise for feeling that way.” In the title winning season, the most common passing duo was Williamson to Lisa Evans (centre half to right wing back), Williamson also finished the season as Arsenal’s most active player on the ball with 1,501 touches to her name, as well as completing the most successful passes in the opposition’s half (591). She also made more interceptions than any other player in Arsenal’s squad (30).

Williamson was involved in the England squad that won in the 2019 SheBelieves Cup in USA, playing the final game, a 3–0 win over Japan.

In May 2019, Ian Wright announced on social media that Williamson had been selected for the 2019 FIFA Women’s World Cup squad and would be on the plane to France. On being selected for the squad, Williamson said, “After the World Cup in Canada four years ago that’s what I set my sights on and all I’ve ever wanted to do is be a part of this team. I can’t be more thankful because I think I’m the luckiest girl in the world.” Williamson made her World Cup debut from the bench in the Round of 16 match against Cameroon on 23 June 2019 in Valenciennes.

Williamson scored her first England goal, an 86th-minute winner, in a 3–2 friendly win against the Czech Republic on 12 November 2019.