The native of Bloomfield Hills, Michigan, was selected by the St. Louis Blues in the third round (No. 67) of the 2012 NHL Draft after scoring 42 goals in 29 games at Brother Rice High School. He played one season with Chicago of the United States Hockey League and three at Michigan State University before signing a two-year, entry-level contract with the Blues.

MacEachern had 11 and 10 points with Chicago of the American Hockey League in his first two pro seasons. He began 2018-19 with San Antonio, the Blues' new AHL affiliate, and had 16 points (nine goals, seven assists) in 33 games before being called up by St. Louis. MacEachern made his NHL debut against the Montreal Canadiens on Jan. 10, 2019, scored his first NHL goal 11 days later against the Los Angeles Kings and ended the season with five points (three goals, two assists) in 29 games.