As a true freshman, Schwellenbach started 44 of Nebraska's games and batted .275 with five home runs and 22 RBIs.[5] He hit .295 in 15 games as a sophomore before the season was cut short due to the coronavirus pandemic.[6] Following the season he played collegiate summer baseball for the Traverse City Pit Spitters of the Northwoods League, where he hit for a .356 average over 22 games.[7][8] After only playing as a position player in his first two collegiate seasons, Schwellenbach was added as a relief pitcher.[9] As a junior, he was named Big Ten Player of the Year and was a second team All-American by the Collegiate Baseball Newspaper.[10][11] He was also named a semifinalist for the Golden Spikes Award.[12][13] In the NCAA Fayetteville Regional of 2021 NCAA Division I baseball tournament, Schwellenbach pitched 4+2⁄3 innings of scoreless relief and drove in the game-tying run in a 5–3 win over Arkansas to force a deciding game 7.[14] He was named the winner of the John Olerud Award as the nation's best two-way player after finishing the season with a .284 batting average and a .403 on-base percentage with six home runs and 40 RBIs while also posting a 3–1 record with ten saves and a 0.57 ERA over 18 pitching appearances.[15][16]
Schwellenbach split the beginning of the 2024 season with the High–A Rome Emperors and Double–A Mississippi Braves, accumulating a 1.80 ERA with 51 strikeouts across 8 starts. On May 29, 2024, Schwellenbach was selected to the 40-man roster and promoted to the major leagues for the first time.[23] He made his MLB debut later that day against the Washington Nationals, yielding three earned runs on five hits in five innings and earning the loss.[24]