The 22-year-old has signed a contract running until 2029, with an option for a further year, in a deal that gives Mechelen their first defensive addition of the summer transfer window.
Born in Nottingham to former professional footballer Jon Olav Hjelde, Leo moved to Celtic's academy at 15 and signed his first professional contract with the Scottish club, where comparisons to Virgil van Dijk followed him as a teenager despite never making a senior appearance for the first team.
His route to regular football instead went through loan spells and permanent moves across England and Scotland. Hjelde first gained senior experience on loan at Ross County in 2021, playing both as a centre-back and at left-back, before moving to Leeds United, where he made his Premier League debut.
A loan at Rotherham United followed in 2023 to build further experience in the Championship, ahead of a permanent transfer to Sunderland in January 2024. He went on to make 30 appearances for the Black Cats, including 19 during their promotion-winning 2024-25 campaign, before a loan at Sheffield United in the second half of last season yielded a further eight appearances in the Championship.
Sporting director Tom Caluwé said Hjelde's experience despite his age made him an ideal fit for the club's needs.
"Hjelde is our first defensive signing of this summer's window. I am confident that Leo has the qualities to be an asset for our team. He already has plenty of experience," Caluwé said. He also indicated the club's transfer business was far from complete. "We're not done yet. We are working on several concrete deals to strengthen the squad further."
Hjelde has also represented Norway at Under-21 level, earning 21 caps for the age group, experience that factored into Mechelen's decision to bring him to the Jupiler Pro League as they look to add competition and depth across their back line for the season ahead.