After signing, Warner was assigned to the Gulf Coast League Cardinals and was briefly elevated to the High–APalm Beach Cardinals due to a shortage of relief pitchers, pitching three scoreless innings and earning a win in relief in his only appearance. He was later promoted to the Single–APeoria Chiefs of the Midwest League.[4] Warner began the 2018 season in the Florida State League with Palm Beach before earning a promotion to the Double-ASpringfield Cardinals after 12 starts with a 3–3 record and a 3.41 ERA.[5] Warner was promoted a second time to the Memphis Redbirds of the Triple-APacific Coast League.[6] He returned to Springfield in 2019 and was named a Texas League All-Star before being promoted to Memphis for the rest of the 2019 season.[7] Warner did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic, but was later assigned to the Cardinals' Alternate Training Site.[6] For the 2021 season, he returned to Memphis.[8] Over 41 appearances for the season, he went 7–3 with a 3.34 ERA and 75 strikeouts over 72+2⁄3 innings.[9] On March 31, 2022, Warner was released by the Cardinals organization.[10]
Seattle Mariners
On April 4, 2022, Warner signed a minor league contract with the Seattle Mariners organization.[11] Warner made 33 appearances (12 starts) for the Triple-A Tacoma Rainiers, logging a 5–3 record and 5.21 ERA with 72 strikeouts in 86.1 innings pitched. He became a free agent following the season on November 10.[12]
Chicago White Sox
On February 28, 2023, Warner signed a minor league contract with the Chicago White Sox organization.[13] He made only three appearances for the Double–A Birmingham Barons, recording a 3.68 ERA with 8 strikeouts across 7+1⁄3 innings pitched. Warner elected free agency following the season on November 6.[14]