Nairn played club soccer with the Freestate Shooters U-18 team, which won the WAGS Championship and Maryland State Cup in 2004 and the U.S. Club Soccer Championship in 2007. She was also a four-year captain for the Region I Olympic Development Program (ODP) team and won the 2006 Regional ODP National Championship.

In January 2008, during her senior year in high school, Nairn was called up to the United States women’s national soccer team and was the youngest member to attend training camp. The same year, Soccer Buzz named her the number three recruit in the country.

In 2010, Nairn played for the Washington Freedom Futures in the W-League. In 2011, Nairn signed with D.C. United Women in the W-League. She made four appearances, scoring three goals and providing one assist notching seven points for the season. She scored twice in a game against the New York Magic leading the team to its first win in franchise history.

As a senior in 2012, Nairn earned her first ever Big Ten Midfielder of the Year and first team NSCAA All-American honor. She finished her career with the second highest number of all-time assists in program history and doubled her career goal total (second highest in the conference) with 17 goals. She was one of two players in the 2012 College Cup with double-digit goals and assists. Nairn scored an exciting golden goal against Florida State in the national semifinals.

In 2012, she played for the NCAA-compliant semi-professional club, ASA Chesapeake Charge, in WPSL Elite. She played in three games and scored one goal.

In 2013, Nairn was one of three finalists for the Hermann Trophy coming in second to University of North Carolina midfielder and future Washington Spirit teammate, Crystal Dunn.

On January 18, 2013, Nairn was drafted to the Seattle Reign FC as the seventh pick (first round) of the 2013 NWSL Draft. On her signing, Nairn said, “It’s a great opportunity, like I said before, to represent Seattle and where I’ve come from – Penn State, and all the teams I played for before. To represent them on a professional stage is a dream come true. I’m excited to get to work.” Nairn scored the first regular season goal for the Reign during the team’s first league match against Chicago Red Stars on April 14, 2013 with a diving header on a cross from forward, Liz Bogus. Nairn led the team in assists and was a leading-scorer. She finished the regular season with five assists and three goals. Nairn returned to the Reign for the 2017 NWSL season. She appeared in all 23 matches, having 21 stars.

On November 20, 2013 the Seattle Reign traded Nairn to the Washington Spirit for the discovery rights to Scottish international Kim Little. In 2014, she scored six goals and took 52 out of 76 corner kicks for the Spirit. In 2015, Nairn scored five goals and recorded two assists. In her third year with the team, the midfielder was voted Spirit MVP for her contributions to the first ever NWSL Championship appearance for the Spirit. She played every match for the Spirit in 2016, starting 18 in the regular season and both post season matches, and scored three goals. Her goal against the Houston Dash on August 18 earned her the NWSL Goal of the Week honors in Week 16. Nairn’s most memorable assist of the season came in the semifinal on Francisca Ordega’s game winning goal to send the Spirit to the NWSL Championship.

In September 2014, Nairn went on loan to the Melbourne Victory for the 2014 W-League season. She returned for the 2015-16 season and 2016-17 season. After taking a season off she returned to the Victory for her fourth loan spell with the club in the 2018-19 W-League season. She won the W-League Premiership in 2018-19. She subsequently received the Julie Dolan Medal, becoming the first international player to receive Australia’s W-League player of the year award.

In 2017, Nairn came out as gay on her personal blog, “Finding my Euphoria”. In her post “What Even is Normal” she stated:

On February 26, 2019, Nairn was traded to the Houston Dash for exchange for an international slot and draft picks.