Leko rose through the ranks at West Bromwich Albion, where he joined the youth academy at the age of 11. He made his first-team debut for the club in 2015 at the age of 16, becoming the first player born in 1999 to play in the Premier League. Leko went on to make a total of 21 appearances for West Brom before moving on loan to Bristol City, Charlton Athletic, and Birmingham City to gain more first-team experience.