Both Americans played a major role in the 2-0 victory over rival 1.FC Köln in front of 51,114 spectators.
The Nov. 10 showdown at Seattle marks the third MLS Cup meeting in four years between the teams.
The Sounders host Toronto FC in their third final in four years.
Before Mathis scored in the 2002 World Cup and spent over a decade in Major League Soccer, he starred for the Gamecocks.
Kerr informed the Red Stars on Friday of her intention to move.
The NWSL’s World Cup boost created a foundation for lasting success.
The league has increased the salary cap by 19.33% and introduced allocation money.
While FC Dallas has been a success in terms of player development, the club’s attendance remains among the league’s worst
12 players are under contract for the 2020 campaign, 11 have options yet to be determined and 5 are out of contract.
Büttner, 30, joins the Revolution after stints with Vitesse, Manchester United, Dynamo Moscow, and Anderlecht