Prior to the start of the National Women's Soccer League's inaugural season, Mathias was selected 22nd overall in the 2013 NWSL Supplemental Draft by FC Kansas City.

She scored two goals: one against the Portland Thorns on June 30 resulting in a 2–0 win for Kansas City and the second in the ninetieth minute against Boston Breakers on July 24.

[9] In November 2014, Mathias was traded to Seattle Reign FC in exchange for Kate Deines and two 2015 draft picks.

[24] During the first few months of the season, a number of offensive players became unavailable due to injury including Manon Melis,[25] Jess Fishlock[26] and Megan Rapinoe.

[27] Seattle finished the regular season in fifth place with a 8–6–8 record, narrowly missing a playoff spot by two points.

[28] Returning to the club for the 2017 season, Mathias scored a goal in the 39th minute in a match against the Portland Thorns FC.

Mathias was an important part of the backline that broke the record for fewest goals conceded, and won the NWSL Shield.

[34] Despite being sidelined during the entire 2023 season due to injury, Angel City announced on November 15, 2023, they would exercise their one-year option and extend her contract through the end of 2024.

[35] Mathias made her long-awaited debut for Angel City on March 17, 2024, coming on a substitute in the home opener against Bay FC.

[39] She received her first cap for United States women's national soccer team on June 12, 2018, in a friendly against China PR in Cleveland.

[43] FC Kansas City Seattle Reign North Carolina Courage United States Individual