Leko joined West Bromwich Albion at the age of 11.[6] According to the club's academy manager, Mark Harrison, he was "a bit of a rough diamond", but by the under-14s he had developed an understanding of the game and was showing considerable creative ability.[4] He was included in the first team's 2015 pre-season tour of Austria, during which he came off the bench in a 3–1 loss against Red Bull Salzburg.[7]
Leko made his competitive debut on 23 September 2015 as a 77th-minute substitute in a League Cup defeat to Norwich City.[8][9] On 2 April 2016, he was named in the matchday squad against Sunderland, and came on for Saido Berahino in the 78th minute to make his Premier League debut at the age of 16 years and 344 days.[10] When he took the field on 30 April against West Ham United, Leko became the first player born in 1999 to start a Premier League match.[11] In the last match of the season, a 1–1 draw with Liverpool, he set up Salomón Rondón's goal with a solo run.[12] Before the match, Leko received the club's Young Player of the Season award.[13] On 28 June, he agreed terms on his first professional contract, a three-year deal lasting to June 2019.[14]
Leko was one of 98 nominations for the European Golden Boy award for 2017, but did not make the 24-man shortlist.[17]
West Bromwich Albion were relegated to the Championship in 2018, but Leko played little during the 2018–19 season.[18]
Bristol City (loan)
Leko made one substitute appearance for West Brom, in the 2017–18 EFL Cup,[19] before joining Championship club Bristol City on 28 August on a season-long loan.[20] He was recalled in January 2018 after 11 goalless appearances, so that he could by assessed by new manager Alan Pardew.[21]
Charlton Athletic (first loan)
On 8 August 2019, Leko and teammate Sam Field joined Charlton Athletic on a season-long loan.[22] He scored his first goal against Reading on 31 August. Having scored five goals from 21 appearances,[23] Leko suffered an ACL injury during the match against Queens Park Rangers on 23 December which ruled him out for the remainder of the season. He returned to his parent club for treatment.[24]
Leko made a complaint to the Football Association about an incident of racial abuse by Leeds United goalkeeper Kiko Casilla in a match in September. Casilla was found guilty by an independent panel in February 2020 and received an eight-match ban and a £60,000 fine.[25] Leko said afterwards that he had been disappointed both by the delay in resolving the issue and by the lack of support from the Professional Footballers' Association (PFA) and from such anti-racism organisations as Kick It Out and Show Racism the Red Card.[26]
Birmingham City
Leko signed a three-year contract with Birmingham on 28 August 2020. The fee was undisclosed,[27] but reported as £1 million.[28] After ten months out with the ACL injury,[29] he finally made his Birmingham City debut on 17 October, as a substitute in a 1–0 defeat at home to Sheffield Wednesday.[30]
Charlton Athletic (second loan)
On 31 August 2021, having made 41 competitive appearances without scoring, Leko re-joined, now, League One club Charlton Athletic on a season-long loan.[31] He made his second debut for the club on 11 September, as a half-time substitute with Charlton two goals down at home to Cheltenham Town, and scored after 59 minutes; the match ended 1–2.[32]
Return to Birmingham City
Upon his return to Birmingham he scored his first goal for the club in an EFL Cup tie against Norwich City on 9 August 2022.[33]
Milton Keynes Dons
Leko signed for League One club Milton Keynes Dons on 13 January 2023 for an undisclosed fee.[34] He made his debut on 21 January 2023, providing an assist in a 2–1 away win over Forest Green Rovers.[35] Leko scored his first goal for the club on 18 February 2023, scoring a 25-yard strike in a 5–2 defeat away to Sheffield Wednesday.[36]
Leko joined League One club Burton Albion on 26 January 2024 on loan for the rest of the season.[37] He made his debut the next day as a half-time substitute away to Cambridge United, but lasted just five minutes before having to leave the field due to another ACL injury - the second of his career. The injury ruled Leko out for the remainder of the season.[38][39]
Leko received his first call-up to the England under-18 team for friendlies against Italy and Israel in September 2016;[42] he appeared in both games.[43] He made four friendly appearances for the under-19s in 2017, but played no competitive matches at that level.[44]