Tomori joined Chelsea at under-8 level and progressed through the club's academy system. He was part of the Chelsea youth side which recorded back to back triumphs in both the UEFA Youth League and the FA Youth Cup in 2015 and 2016. On 11 May 2016, Tomori was named upon the first-team substitute bench along with fellow academy players Tammy Abraham and Kasey Palmer, in Chelsea's 1–1 draw with Liverpool. However, he failed to make an appearance at Anfield. On 15 May 2016, in Chelsea's final game of the 2015/16 campaign, Tomori made his professional debut in a 1–1 draw with Premier League champions Leicester City, replacing Branislav Ivanović in the 60th minute. Although Tomori was included in the United States pre-season tour, he did not make a single appearance. On 1 August 2016, Tomori signed a new four-year contract ahead of the 2016–17 campaign. On 12 August 2016, Tomori was handed the number 33 shirt prior to the season's beginning, switching from his previous number 43.
On 23 January 2017, Tomori joined Championship side Brighton & Hove Albionon loan for the remainder of the 2016–17 campaign. Five days later, Tomori made his Brighton debut in a 3–1 away defeat against National League side Lincoln City in the FA Cup fourth round, in which he scored an own goal to give Lincoln the lead. On 18 February 2017, Tomori made his league debut for Brighton, in their 2–0 away victory against Barnsley, replacing Anthony Knockaert in stoppage time of the second half. On 18 March 2017, Tomori was given his first start for Brighton in their 2–0 away defeat against Leeds United, featuring for the entire 90 minutes.
On 31 August 2017, Tomori joined Championship side Hull City on a season long loan deal. He made his debut on 13 September 2017, in a 2–1 defeat away to Fulham.
On 6 August2018, Tomori joined Championship side Derby County on a season long loan. He made his debut on 11 August in a 1–4 loss against Leeds United. His time at the club saw him named the club's "Player of the Year".
2019–present: Return to Chelsea
Following the expiration of his loan, Tomori returned to Chelsea where he was given the No. 29 shirt. On 31 August 2019, Tomori made his first start for Chelsea against Sheffield United, which ended a 2–2 draw at Stamford Bridge. He scored his first goal for Chelsea on 14 September 2019, opening the scoring with long-range curler from outside the box, in a 5–2 away win over Wolves. He scored a header against former club Hull City to help Chelsea to a 2–1 win in the fourth round of the FA Cup at the KCOM Stadium on 25 January.
International career
Tomori was eligible to represent Nigeria at international level through his parents, Canada through his birth in Calgary and England through his upbringing having lived in the country since he was a toddler.
Canadian national youth team
On 27 March 2016, Tomori captained Canada U20 to a 2–1 victory over England U20 on his third appearance for the nation.
England national youth teams
On 16 May 2016, in the next international break and a day after making his professional club debut, Tomori was called up to the England U19 squad. On 4 June 2016, Tomori made his England U19 debut in a 2–0 defeat against Mexico U20s, playing the full 90 minutes. Tomori was also part of the England 2016 UEFA European Under-19 Championship squad which reached the semi-finals before being knocked out by Italy.
Tomori was selected for the England under-20 team in the 2017 FIFA U-20 World Cup. Tomori however scored an own goal in England's second group game against Guinea. Nevertheless, Tomori would help England beat Venezuela in the final 1–0, which is England's first win in a global tournament since their World Cup victory of 1966.
On 3 October 2019, Tomori received his first call-up to the England senior squad for forthcoming Euro 2020 qualifying matches. He later said he was committed to playing for England, following interest from Nigeria and Canada; he had previously represented Canada at youth level. Tomori made his debut for England on 17 November 2019 against Kosovo.
