In January 2013, Heather O'Reilly, Heather Mitts and Leroux were the three United States national team players allocated to the Boston Breakers for the inaugural season of the National Women's Soccer League (NWSL). She scored her first goal of the season in the 91st minute of the Breakers' season opener against the Washington Spirit, tying the game 1–1 in stoppage time. 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. On July 3, she scored another equalizer against the Seattle Reign FC, resulting in a 1–1 draw. 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.
On November 18, 2013, it was announced that Leroux had been traded to the Seattle Reign FC for the 2014 NWSL season in exchange for Kristie Mewis, Michelle Betos and the Reign's first two picks in the 2015 draft. Ebullient Reign FC head coach Laura Harvey hailed the signing of Leroux: "One of the world's best forwards. She is a young, extraordinary talent with unlimited upside. Sydney will be the consistent goal-scoring threat that we struggled to find last season."
While playing in a sold-out friendly match against Canada at BMO Field in Toronto, Ontario in June 2013, Leroux was booed by Canadian fans throughout the match. After scoring during stoppage time to bring the score to 3–0 in favor of the U.S., Leroux celebrated her goal by hushing the crowd and pointing to the U.S. emblem on her jersey which further inflamed many fans in the crowd. 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. #america." The U.S. Soccer Federation announced that Leroux had "endured abuse both verbally and in social media" since switching to the U.S. national soccer team in 2008; however, Leroux later clarified that no such incidents took place at BMO Field. "My tweet from this morning wasn't in response to anything from yesterday's match at BMO Field. In fact, the atmosphere at the stadium was a positive step forward for women's soccer. 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. That is what prompted my response in the heat of the moment."
During a friendly against Mexico in September 2013, Leroux scored a hat trick in nine minutes, helping the U.S. win 7–0.
Leroux has appeared in several advertisements and promotional pieces for Nike. 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. In June 2014, she signed an endorsement deal with Nestlé Nesquik. The same month, she made a cameo appearance in a commercial for Beats by Dre in preparation for the 2014 FIFA World Cup. 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.
Leroux was one of 21 professional athletes featured in ESPN's The Body Issue in 2013. She appeared semi-nude on one of eight covers for the magazine. Of the experience she said,