Born in Canada to a Canadian mother (Sandi Leroux) and an American father (Ray Chadwick), Sydney came up through the Canadian system and represented Canada at various youth levels until she chose to play for the United States women's national under-20 soccer team starting in 2008 and later the U.S. senior national team starting in 2012.

Leroux has earned over 75 caps with the senior national team and was part of the Americans' winning squads at the 2012 London Olympics and 2015 Women's World Cup.

[12] Leroux played three seasons of club soccer with Coquitlam City Wild, helping the team to Provincial Cup Championships in 2003, 2004 and 2005.

[8][15] To pursue her goal, she moved to Scottsdale, Arizona at the age of 15 and attended Horizon High School during her junior and senior years while living with multiple host families.

[16][17][18] Despite having a challenging time adjusting to life without her family and friends, she helped lead Sereno Soccer Club to state titles in 2007 and 2008.

[36] During the summer of 2012, Leroux joined fellow U.S. national team players Hope Solo, Alex Morgan, Stephanie Cox, and Megan Rapinoe on the Seattle Sounders Women in the W-League.

[37] Sounders Women head coach, Michelle French, said of the signing, "Sydney is the consummate goal scorer with an incredible attacking attitude.

"[38] Due to her national team commitments and preparation for the 2012 Summer Olympics,[39] Leroux made only two regular season appearances for the club.

[45] Leroux scored the league's first ever hat trick during the Breakers' 4–1 win against the Chicago Red Stars on May 4 and was subsequently named NWSL Player of the Week.

[47] A few days later on July 6, she scored two goals against the Portland Thorns FC resulting in a 2–0 win for the Breakers in front of 12,436 fans at Jeld-Wen Field.

Ebullient Reign FC head coach Laura Harvey hailed the signing of Leroux: "One of the world's best forwards.

[57] Due to her participation in the 2015 FIFA Women's World Cup and an ankle injury, Leroux played only three games for the Flash.

[64] She returned to playing during the 2019 season, making an 86th-minute substitute appearance against Sky Blue FC on September 29, just three months after giving birth.

[69] During the 2023 season, Leroux scored her first goal for Angel City after returning from surgery and recovery in a 2–1 loss against the Chicago Red Stars.

[71] Angel City finished in fifth place during the regular season and advanced to the playoffs for the first time[72] where they were eliminated by OL Reign in the quarter final match.

[74] On May 18, 2024, in an away match against Washington Spirit which ended as 2–4 defeat, Leroux scored her third goal of the season, her 44th in the NWSL overall, surpassing Lloyd and solidifying her spot in the top 10 all-time scorers.

[77] Leroux scored the 100th goal in Angel City's club history on October 20, 2024, an equalizer to secure a 1–1 draw in the final home match of the season against Utah Royals.

[8] Playing for Canada at the age of 14, she was the youngest individual to participate in the 2004 FIFA U-19 Women's World Cup, held in Thailand.

[83] Leroux played at the 2012 CONCACAF Women's Olympic Qualifying Tournament as a member of the United States senior national team.

[87] Leroux's performance tied the record for goals scored in a single match by one player in a CONCACAF Olympic Qualifying Tournament.

She also tied the single-game record for the national team,[88] equaling previous performances by her teammates Amy Rodriguez and Abby Wambach in 2012 and 2004 respectively.

[91] During the 2012 Algarve Cup in Portugal, Leroux scored the team's fifth goal in the 93rd minute of the U.S.' first group stage match against Denmark, in which the U.S. won 5–0.

[101] After the game, she stated via Twitter, "When you chant racial slurs, taunt me and talk about my family don't be mad when I shush you and show pride in what I represent.

Unfortunately, the type of abuse I have received in the past and via social media for my decision to play for the United States is a step backwards.

[111] Leroux has not received a call-up since the 2017 Tournament of Nations, and she was not listed on the 35 player provisional roster for the 2018 CONCACAF Women's Championship.

[121] In March 2019, at six months pregnant, Leroux was photographed participating in light preseason training with the image causing controversy; drawing both criticism and praise.

[128][129] In December 2013, she and national teammate Alex Morgan were featured in Nike's "Winning in a Winter Wonderland" commercial along with other professional athletes including Robinson Canó, Justin Tuck, and Julia Mancuso.

[132] In June 2014, she became the first female endorser for the sports drink company BODYARMOR, joining fellow professional athletes Richard Sherman, Kevin Love, and James Harden.

[133][134] In 2022, Sydney partnered with DIRECTV [135] to release the Undercover Coach video,[136] promoting the brand's partnership with LeagueSide and reminding us the future is female.

Leroux was the focus of an ESPN feature entitled Living Her Dream, which profiled her evolution as an international soccer player.