Fleming played college soccer in the United States for the UCLA Bruins from 2016 to 2019, before turning professional and spending four years with English club Chelsea.

She was selected the Top Drawer Soccer Freshman of the Year, and also earned first-team NSCAA All-Pacific Region, All-Pac-12 and All-Freshman acclaim.

After helping UCLA reach the College Cup final and scoring a goal in the championship match, she was selected as a finalist for the Hermann Trophy[17] and chosen as one of four nominees for the Class of 2018 Honda Sports Award for soccer.

[18] Although she missed nearly half of her junior season while on national team duty, Fleming still earned first-team All-Pac-12 honours for the third year in a row and was a second-team All-West Region honouree after scoring 15 points on 5 goals and 5 assists.

[19] As a senior, Fleming played in a defensive midfield role, finishing the season with 3 goals and 4 assists in 22 appearances, for a total of 10 points.

[28] On March 14, she played the full 90 minutes as the Blues won the 2021 FA Women's League Cup Final against Bristol City at Vicarage Road.

[31] Fleming would later cite the transition to the professional scene as a challenge, in particular the need to raise her game in order to vie for playing time.

[32] Beginning the 2021–22 FA WSL, Fleming scored her first goal for Chelsea in a 6–1 victory against Manchester United at Leigh Sports Village on September 26, 2021.

[33] Three days later, she got her first assist for the club in the 2020–21 Women's FA Cup quarter-final against Birmingham City, setting up Fran Kirby in the 72nd minute.

[35] On October 31, she assisted Bethany England in a 3–0 victory against Manchester City at the Academy Stadium, helping Chelsea reach the 2021 Women's FA Cup Final.

[37] The 2021–22 WSL season saw Fleming's usage by the club increase significantly, featuring in 21 of 22 games, 11 of those as starts, and tallying six goals and four assists.

She won the league title with Chelsea for the second time, before playing in her second consecutive run with the club to the 2022 Women's FA Cup Final.

[52] At the 2014 U-17 World Cup in Costa Rica, Fleming navigated Canada out of the group of death (which included Germany, North Korea, and Ghana).

[56][57] Fleming also represented Canada at the 2015 Pan American Games, where the Canadian Soccer Association fielded an Olympic Developmental under-23 squad.

[59] In December 2013, following her performances at the CONCACAF Women's U-17 Championship, Fleming received her first call-up to the senior national team, ahead of the 2013 Torneio Internacional de Brasília.

[60][61] On December 15, she made her senior international debut at the age of 15 years and 278 days, when she replaced Brittany Baxter in the 72nd minute of a 1–0 defeat to Chile at the Estádio Nacional Mané Garrincha.

[4] In early 2015, Fleming moved to Vancouver to train full-time with the national team ahead of the 2015 FIFA World Cup.

[70] On June 15, she started Canada's final group stage match of the World Cup against Netherlands in front of 45,420 fans at Olympic Stadium.

[75] Fleming made her 50th senior appearance on March 7, 2018, scoring her fifth international goal in a 3–0 win over South Korea at the 2018 Algarve Cup.

[76] She was by this point established as one of the most promising younger players on a team, at a time when commentators were increasingly contemplating its future upon the eventual retirement of longtime captain Sinclair.

[83][84] In the August 2, 2021, semi-final match with the United States, Fleming scored the only goal via a penalty kick, resulting in a 1–0 win that sent Canada to its first ever Olympic final.

[96] Fleming was widely expected to be a central role on the team in the tournament, with many commentators identifying her as Canada's most important player.

"[42] However, as a result of a practice injury, Fleming was unable to play in the opening match against Nigeria, which ended in a 0–0 draw, with her absence being widely noted as a major weakness.

[99][100] She returned to play the full 90 minutes in Canada's 2–1 victory over Ireland, wearing the captain's armband in the first half prior to Sinclair's substituting on.