She previously played college soccer for NCAA Division I programs Notre Dame Fighting Irish and Florida Gators before playing professionally for the Boston Breakers, Chicago Red Stars, Western New York Flash, Sky Blue FC, Seattle Reign and Portland Thorns in the NWSL, Swiss club FC Zürich in the Nationalliga A, and West Ham United and Manchester United of the English Women's Super League.

Leon made her senior international debut in 2012, and has represented Canada at two FIFA Women's World Cup tournaments and won Olympic gold at Tokyo 2020.

In the 2010 College Cup final she came off the bench to score the lone goal in a 1–0 victory over the Stanford Cardinal and was named to the All-Tournament Team.

Leon scored five goals including four game-winners for the Gators and helped the team to Southeastern Conference regular season and tournament titles.

The Breakers ceased operations prior to the start of the 2018 season and the players were redistributed within the league via the 2018 NWSL Dispersal Draft.

On June 12, 2018, Leon was traded to the Seattle Reign FC in exchange for a fourth-round pick in the 2019 NWSL College Draft.

[20] Leon made six appearances for Seattle in 2018 including one start before the club opted not to retain her NWSL rights ahead of the 2019 season.

[21] On January 12, 2019, Leon signed for West Ham United midway through the team's debut Women's Super League season.

[22] In her debut campaign she made 10 appearances in all competitions, scoring three goals including two in a 3–1 win over Blackburn Rovers in the fourth round of the FA Cup.

On June 29, 2022, West Ham announced Leon had decided to leave the club upon the expiration of her contract that summer after three and a half seasons.

She was reportedly the subject of a loan move from A.C. Milan in January and then later offered to several WSL clubs but United stalled and none of these deals could be completed in time.

[26] On April 13, 2023, still searching for playing time having made two appearances since the turn of the year, Leon joined National Women's Soccer League club Portland Thorns on loan for the remainder of the 2022–23 WSL season, until June 30, 2023.

Canada's second-place finish qualified the team for the 2012 FIFA U-20 Women's World Cup and Leon was recalled to the squad for the tournament in Japan.

[42] Leon also played a crucial role in Canada's two September friendly matches against Australia, held in anticipation of the following year's World Cup that the latter would co-host.

[43][44] After what was widely deemed a difficult club season with limited playing time, there were questions about Leon's form going into the 2023 FIFA Women's World Cup.

[47] Following the team's disappointment at the World Cup, Leon made her 100th international appearance in the first match of the two-part CONCACAF Olympic qualification playoff against Jamaica.

[49][50] Canada reached the semi-final against the United States, and with less than a minute remaining in extra time with the Americans up 2–1, Leon was tasked with taking a penalty.

[52][53] Media commentary subsequently remarked that Leon had "staked a claim" to the team's starting striker role following the retirement of Christine Sinclair.