Bellingham took up a two-year scholarship with Birmingham City to begin in July 2019. He was part of the first-team squad that spent a week in Portugal as part of their pre-season training, played and scored in pre-season friendlies, and was given squad number 22 for the 2019–20 season. On 6 August 2019, Bellingham became Birmingham City's youngest ever first-team player – at the age of 16 years, 38 days, beating the record of 16 years 139 days set by Trevor Francis in 1970 – when he started the EFL Cup first round visit to Portsmouth. He played for 80 minutes in the 3–0 defeat, and was the Birmingham Mail's man of the match. He made his first Football League appearance 19 days later, as a second-half substitute in a 3–0 defeat away to Swansea City, and his home debut on 31 August against Stoke City. Entering the game after half an hour to replace the injured Jefferson Montero, Bellingham went on to score the winning goal – albeit via a generous deflection – as Birmingham came back from 1–0 down to beat Stoke 2–1, and thus became Birmingham's youngest ever goalscorer, at the age of 16 years and 63 days. He started the next match, away to Charlton Athletic two weeks later, and scored the only goal from Kerim Mrabti's cutback.