He started 2016 with the Stockton Ports and was promoted to the Double-A Midland RockHounds during the season;[4] he went a combined 7–6 record with a 3.29 ERA.

[6] On January 29, 2018, Fillmyer and Jesse Hahn were traded to the Kansas City Royals in exchange for Brandon Moss and Ryan Buchter.

[8] He made his major league debut on June 24, pitching four innings of mop-up duty in an 11–3 loss against the Houston Astros.

He also collected his first Major League hit on September 26, an RBI double off of Cincinnati Reds reliever Matt Wisler.

[13] Fillmyer pitched six innings of one-run ball with nine strikeouts in the first game of the season for Lancaster, his only appearance with the club.

[16] On November 15, 2021, Fillmyer signed a minor league contract with the San Diego Padres organization.