Ethan Lanthier attended Northfield High School in Northfield, Minnesota, where he played varsity baseball for three years and varsity football for two. He was named to various honorary teams in both sports but excelled more on the diamond. In his senior year, he was named 2020-2021 Northfield Men’s Athlete of the Year and earned Class AAA All-State honors for posting a 1.05 ERA in 20.0 innings on the mound with 38 strikeouts while hitting .350. After graduating, he went undrafted in the 2021 MLB Draft and attended St. Cloud State University in St. Cloud, Minnesota.
In his freshman year, Lanthier appeared in 15 games for the Huskies, opening two, and posted a 2.