Born and raised in Forest Lake, Minnesota, Wallner played three years of college baseball at the University of Southern Mississippi.

[4][5] Following his high school graduation, Wallner was selected by the Minnesota Twins in the 32nd round of the 2016 Major League Baseball draft.

[8][9] He also garnered Freshman Hitter of the Year honors from the National Collegiate Baseball Writers Association.

[10] In addition, he was named to the C-USA All-Freshman Team as well as the C-USA Second Team, and was also named a Freshman All-American by multiple outlets including the Collegiate Baseball Newspaper and the National Collegiate Baseball Writers Association.

[11][12] As a sophomore in 2018, Wallner batted .351 with 16 home runs and 67 RBIs in 62 games while recording a 7.98 ERA in 14+2⁄3 innings.

[22] Wallner batted .323 with 23 home runs and sixty RBIs for the season, earning a spot on the C-USA First Team.

[38] Wallner was assigned to the Wichita Wind Surge of the Double-A Texas League to begin the 2022 season.

[41] Over 128 games played between Wichita and St. Paul, he slashed .277/.412/.541 in 458 at bats with 27 home runs, 95 RBIs, and 32 doubles, and was third in the minor leagues with 97 walks.