Liam Greentree was born in Oshawa, Ontario, Canada. As a youngster he played for the Markham Majors U16 AAA team in the GTHL U16 League. He played in 22 games and tallied 13 goals and 12 assists. Greentree was drafted at #34 by the Windsor Spitfires of the OHL in 2022.
In 2022-2023, Greentree scored 25 goals and 20 assists in 61 games for Windsor. Greentree was named Spitfires captain before the start of the 2023-2024 season. Along with showing leadership skills, he went on to show the hockey world his ability to be a high end sniper. He scored 36 goals and added 54 assists in 64 games. During the year he also participated in the WJC-18 games for Canada U18 and had 2 goals and 2 assists in 7 games.