On July 31, 2015, the Blue Jays traded Brentz, Rob Rasmussen, and Nick Wells to the Seattle Mariners in exchange for Mark Lowe.[3][4] He finished the season with the Low–A Everett AquaSox. In 2016, he split the season with Everett and the Single–A Clinton LumberKings, also appearing in two games for the Triple–A Tacoma Rainiers.
On September 1, 2016, the Mariners traded Brentz along with Pedro Vasquez to the Pittsburgh Pirates in exchange for Arquimedes Caminero.[5] Brentz split the 2017 and 2018 season between the High-A Bradenton Marauders and Double-A Altoona Curve.[6] Brentz started the 2019 season with Altoona before being promoted to the Triple-A Indianapolis Indians.[7] On August 16, 2019, the Pirates released Brentz.[8]
On August 20, 2019, Brentz signed a minor league contract with the Kansas City Royals organization.[9] He finished the 2019 season with the Double-A Northwest Arkansas Naturals, hurling 5.1 innings of 3.38 ERA ball with 8.4 K/9.[10] On July 19, 2020, Brentz was added to the Royals 60-man player pool, a replacement for the minor leagues due to the cancellation of the Minor League Baseball season because of the COVID-19 pandemic.[11] Brentz was invited to Spring Training for the Royals in 2021.[12]
On April 1, 2021, Brentz had his contract selected to the 40-man roster, and it was announced that he had made the Royals Opening Day roster.[13] On April 3, 2021, Brentz made his MLB debut in relief against the Texas Rangers, pitching a scoreless 2/3 inning and notching his first major league strikeout, striking out Rangers first baseman Nate Lowe.
Brentz began the 2022 season with Kansas City, posting a 0–3 record and 23.63 ERA with nine strikeouts in eight appearances. On June 12, 2022, Brentz was placed on the 60-day injured list with a left flexor strain.[14] He underwent Tommy John surgery on July 21, ending his season.[15] On November 15, Brentz was designated for assignment by the Royals.[16] On November 18, it was announced that the Royals would not tender Brentz a contract for the 2023 season, making him a free agent.
On March 16, 2023, Brentz signed a two-year contract with the Royals.[17] On August 14, he began a rehab assignment with the rookie–level Arizona Complex League Royals.[18] On August 22, his assignment was moved to Double–A,[19] however he was removed three days later for an unspecified reason.[20] The same day, Brentz was diagnosed with a left lat strain that would end his season prematurely.[21]
Brentz began the 2024 season in the minor leagues, and struggled to a 12.71 ERA in 20 appearances split between Northwest Arkansas and the Triple–A Omaha Storm Chasers. He was designated for assignment by the Royals on June 24, 2024.[22] Brentz cleared waivers and was sent outright to Omaha on June 28.[23]
