In 2022, Lajeunesse began attending the University of British Columbia, where he played for the men's soccer team.[3] While at UBC, he pursued a biology degree.[4] On September 25, 2022, he scored his first goal in a 4-0 victory over the Saskatchewan Huskies.[5] In his rookie season, he was named the Canada West Rookie of the Year and the U Sports Rookie of the Year.[6] In 2023, he was named to both the Canada West and U Sports First Team All-Stars.[7][8]
At the 2023 CPL-U Sports Draft, Lajeunesse was selected in the first round (6th overall) by Pacific FC.[11][12] After attending pre-season with the club, he signed a U-Sports development contract with the club in April 2023, allowing him to maintain his university eligibility.[13] He made his professional debut on April 21 in a 2023 Canadian Championship match against Cavalry FC.[14] He also spent some time on loan with Pacific's League1 British Columbia affiliate Nautsa’mawt FC in 2023.[15] In August 2023, he departed the club to return to university, per the terms of his U Sports contract.[16][17] Pacific subsequently chose to retain his rights for the following season.[18] In April 2024, he signed another U Sports contract with the club for the 2024 season.[19] He again departed the club in August 2024 to return to university with the club retaining his rights for 2025.[20]