He signed with the Mariners and made his professional debut with the Arizona League Mariners where he pitched to a 0–2 record and 1.53 ERA in 11 starts.
Neidert pitched in 2016 for the Clinton LumberKings, posting a 7–3 record with a 2.57 ERA in 19 starts. He spent 2017 with both the Modesto Nuts, with whom he was named a California League All-Star,[5][6][7] and the Arkansas Travelers, going a combined 11–6 with a 3.45 ERA in 25 starts between both teams.[8] He missed the final month of the season due to injury.
Neidert was added to the Marlins 40–man roster following the 2019 season on November 20, 2019.[15] On July 25, 2020, Neidert made his major league debut against the Philadelphia Phillies pitching 2 1/3 scoreless innings. He made 4 appearances for the big league club in 2020, posting a 5.40 ERA with 4 strikeouts. In 2021, Neidert made 8 appearances for the Marlins, tossing 35.2 innings of 4.54 ERA ball with 21 strikeouts. He spent the majority of the year with the Triple-A Jacksonville Jumbo Shrimp, logging a 6-4 record and 3.67 ERA with 52 strikeouts in 68.2 innings pitched.[16]
On April 3, 2022, Neidert was designated for assignment by Miami to create roster space for Tanner Scott and Cole Sulser, who were acquired from the Baltimore Orioles.[17] He cleared waivers and was outrighted to the Triple-A Jacksonville Jumbo Shrimp on April 8. He had his contract selected back to the major league roster on July 30. He only made 1 start for Miami in 2022, allowing 2 runs on 5 hits with 3 strikeouts in 5.0 innings pitched against the New York Mets. On September 6, it was announced that Neidert had undergone knee surgery, and missed the remainder of the season.[18] On November 15, Neidert was designated for assignment. On November 18, he was non tendered and became a free agent.
Chicago Cubs
On December 24, 2022, Neidert signed a minor league deal with the Chicago Cubs.[19] He made 27 appearances (24 starts) for the Triple–A Iowa Cubs, compiling a 5–7 record and 5.64 ERA with 86 strikeouts across 107 innings pitched. Neidert elected free agency following the season on November 6, 2023.[20]