A resident of the
Roebling section in
Florence Township, New Jersey, Fillmyer attended
Florence Township Memorial High School and played
college baseball at
Mercer County Community College. He was drafted by the
Colorado Rockies in the 28th round of the
2013 Major League Baseball draft, but did not sign and returned to Mercer County.
Oakland Athletics He was then drafted by the
Oakland Athletics in the fifth round of the
2014 MLB draft. Fillmyer made his professional debut with the
Arizona League Athletics, going 1–0 with a 2.79 ERA in six games, and pitched 2015 with the
Beloit Snappers, where he posted a 3–13 record with a 4.98 ERA in 23 games. He started 2016 with the
Stockton Ports and was promoted to the Double-A
Midland RockHounds during the season; he went a combined 7–6 record with a 3.29 ERA. In 2017, he pitched for Midland, posting an 11–5 record with a 3.49 ERA. The Athletics added him to their 40-man roster after the season.
Kansas City Royals On January 29, 2018, Fillmyer and
Jesse Hahn were traded to the
Kansas City Royals in exchange for
Brandon Moss and
Ryan Buchter. On June 19, the Royals called up Fillmyer. 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. Fillmyer was permanently made a part of the Royals’ rotation on July 23, where he made 12 starts. He finished the season with a 4.26 ERA. His finest start of the 2018 season came on August 9, when he threw seven shutout innings against the
Chicago Cubs. He also collected his first Major League hit on September 26, an RBI double off of
Cincinnati Reds reliever
Matt Wisler. Fillmyer spent most of the 2019 season in the minors, although he did see some limited major league action. On January 22, 2020, Fillmyer was
designated for assignment to make room for
Alex Gordon on the roster. On August 18, 2020, Fillmyer was released by the Royals organization.
Lancaster Barnstormers On March 3, 2021, Fillmyer signed a minor league contract with the
Arizona Diamondbacks organization On March 30, the Diamondbacks released Fillmyer. On May 3, 2021, Fillmyer signed with the
Lancaster Barnstormers of the
Atlantic League of Professional Baseball. 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.
Cleveland Indians On May 30, 2021, Fillmyer's contract was purchased by the
Cleveland Indians organization and he was assigned to the Triple-A
Columbus Clippers. In 21 games (16 starts) for Columbus, he posted a 4–7 record and 6.18 ERA with 79 strikeouts in 83.0 innings of work. Fillmyer elected free agency following the season on November 7.
San Diego Padres On November 15, 2021, Fillmyer signed a minor league contract with the
San Diego Padres organization. In 5 games for the Triple–A
El Paso Chihuahuas, he struggled a 7.56 ERA with 8 strikeouts in innings pitched. He elected free agency following the season on November 10, 2022. ==Coaching career==