Leo Sahlin Wallenius was born in Skovde, Sweden.
Sahlin Wallenius played junior hockey for Vaxjo and in 2022-2023 he played in 30 games and recorded two goals and six assists.
This season he took his game to the next level for Vaxjo. He finished the year with 11 goals and 31 assists in 43 games. At an under-18 Five Nations event in February, he tied for the team scoring lead with seven points, as the Swedes won all four games en route to winning the tournament.