On January 11, 2013, it was announced that Sinclair would play for the Portland Thorns FC for the inaugural season of the National Women's Soccer League via the NWSL Player Allocation. Playing as team captain, she appeared in 20 games in the 2013 season and tied with Alex Morgan as the top scorer on the team with eight goals. Sinclair was named the league's Player of the Month for the month of April after scoring two goals and serving one assist to help the team secure a 2–0–1 record.