To begin his professional baseball career, Guenther was assigned to the Gulf Coast League Marlins of the Gulf Coast League in 2017. In three starts and four total games with the GCL Marlins, Guenther only allowed one run, five hits, and eleven strikeouts in 11 innings. Guenther was quickly promoted to the Batavia Muckdogs of the New York-Penn League. In 40 innings with the Muckdogs, Guenther posted a 1–4 record with thirty-three strikeouts and a 3.83 ERA. For the 2018 season, Guenther was assigned to the Jupiter Hammerheads of the Florida State League. Early struggles with the Hammerheads had Guenther demoted to the Greensboro Grasshoppers of the Single A South Atlantic League.[6] Guenther also appeared in a rehab game with the GCL Marlins in 2018.[7] In 2019, Guenther appeared with the Clinton LumberKings of the Midwest League and with the Jupiter Hammerheads. Guenther did not appear with a team in 2020 because of the COVID-19 pandemic.
To begin the 2021 season, Guenther was assigned to the Pensacola Blue Wahoos of Double-A South. In 17.2 innings with Pensacola, Guenther posted a 1–0 record with a 1.02 ERA and 28 strikeouts.[8] On June 8, Guenther was promoted to the Jacksonville Jumbo Shrimp of Triple-A East.[9] On August 4, Guenther was called up by the Marlins and made his debut 2 days later.
On April 7, 2022, Guenther underwent Tommy John surgery, ending his 2022 season before it began.[10]
Guenther began the 2024 campaign with Toledo, recording a 3.86 ERA with 46 strikeouts and 3 saves across 44+1⁄3 innings pitched. On August 1, 2024, the Tigers selected Guenther's contract, adding him to their active roster.[13] He made his season debut later that day against the Kansas City Royals, coming in with two outs in the 8th inning in relief of Brenan Hanifee. Guenther finished the 8th inning and continued into the 9th, ultimately giving up 1 hit, 1 earned run, and 0 walks over 1+1⁄3 innings.[14]