Bellingham was born in Stourbridge, West Midlands, in 2003, the eldest son of Denise and Mark Bellingham. Mark works as a sergeant in the West Midlands Police and was a prolific goalscorer in non-League football. Bellingham’s younger brother, Jobe, is a member of Birmingham City F.C.’s academy. Bellingham attended Priory School in Edgbaston, Birmingham.

Bellingham made his England under-15 debut against Turkey in December 2016. In recognition of his captaining that team during the 2017–18 season, he was presented with a Special Achievement Award at the 2018 Birmingham City Academy awards night. By the end of 2018 he had made his first appearance for the England under-16 team, and went on to feature in seven games, score four goals, and captain the team.

Bellingham joined Birmingham City as an under-8. He played for their under-18 team at 14, and made his debut for their under-23 team at the age of 15, on 15 October 2018 away to Nottingham Forest’s U23s. Entering the game after an hour, he scored the only goal in the 87th minute “sliding in to force the ball over the goal line after pressure by Kyle McFarlane on the keeper diverted the ball into his path.” By March 2019, he had three goals from ten development squad appearances, had featured in FourFourTwo’s list of the “50 most exciting teenagers in English football”, and was being mentioned as of interest to major European clubs. He was gradually introduced to the first-team environment while still a schoolboy, increasingly training with the seniors, accompanying them on matchday to observe, and, in March 2019, travelling as the “19th man” for the visit to West Bromwich Albion in the Championship.

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.

He was included in England’s under-17 squad for the Syrenka Cup, a friendly tournament held in September 2019 in preparation for the 2020 European Championship qualifiers the following March. He made his debut as a substitute in England’s opening match of the tournament, a 5–0 win over Finland in which he scored the third goal, and captained the team in their second fixture, in which they came back from a goal behind to beat Austria 4–2 and qualify for the final. Again, Bellingham scored the third goal. He retained the captaincy for the final, in which England beat hosts Poland on penalties following a 2–2 draw, and was named player of the tournament.

He was linked with moves to numerous major clubs in January 2020; on deadline day, Birmingham were reported to have turned down a £20 million bid from Manchester United. Bellingham continued as a first-team regular, and by the time the season was suspended because of the COVID-19 pandemic, he had made 32 league appearances. He remained an integral part of the team once the season resumed behind closed doors, and set up a late equaliser for Lukas Jutkiewicz against Charlton Athletic that made Birmingham’s league position less precarious with two matches still to play. He finished the season with four goals from 44 appearances in all competitions, 41 in the league, as Birmingham avoided relegation despite losing the last match of the season. In appreciation of what Bellingham achieved in such a short time with the first team, the club announced that they would retire his number 22 shirt, “to remember one of our own and to inspire others.” At the EFL Awards, he was named both Championship Apprentice of the Year and EFL Young Player of the Season.

Bellingham made his debut on 14 September 2020 in the starting eleven for the first fixture of Borussia Dortmund’s 2020–21 season, against third-tier MSV Duisburg in the DFB-Pokal, at the age of 17 years, 77 days. After half an hour, he scored the second goal in a 5–0 win, and became not only the club’s youngest goalcorer in the DFB-Pokal, taking six days off Giovanni Reyna’s record, but also their youngest scorer in any competitive match, breaking Nuri Şahin’s record by five days. Five days later, he marked his Bundesliga debut with the assist for Reyna’s opening goal in a 3–0 win over Borussia Mönchengladbach, and was named as Bundesliga Rookie of the Month for September. When Bellingham faced Lazio in the group stage on 20 October, aged 17 years and 113 days, he became the youngest Englishman to start a Champions League match, breaking the record previously set by Phil Foden.

Bellingham received his first call-up to the under-21 squad for European Championship qualifiers against Kosovo and Austria in September 2020. He became the youngest player to appear for England U21 when he came on to replace Tom Davies after 62 minutes of the match against Kosovo on 4 September with England 3–0 ahead, and scored after 85 minutes to complete the 6–0 victory.

On 10 November 2020, Bellingham was called up to the England senior team for the first time following the withdrawals of James Ward-Prowse and Trent Alexander-Arnold through injury. He made his debut in a friendly against the Republic of Ireland at Wembley on 12 November, replacing Mason Mount after 73 minutes of a 3–0 win. At 17 years, 136 days, he became England’s third-youngest full international; only Theo Walcott and Wayne Rooney had appeared at a younger age.