Chalobah made his England under-16 debut at the age of 13 in October 2008, and his England under-17 debut aged 14 in July 2009. He won the 2010 UEFA European Under-17 Championship and later captaining the team at the age of 15. Chalobah made his England under-19 debut in September 2011., and went on to captain them until 8 November 2012 when he was called up to the England under-21 squad for a match against Northern Ireland. Chalobah made his England Under-21 debut on 13 November 2012, replacing Jordan Henderson in the 76th minute.

After initially joining the academy system at Fulham, Chalobah moved to the Chelsea academy at the age of 10. During the 2010–11 season, he featured for Chelsea reserves while still a schoolboy, winning the national championship with the team that season. In 2011–12, Chalobah regularly captained both Chelsea’s reserve and youth teams, and led the 2012 FA Youth Cup winning team. He signed a professional contract with Chelsea in January 2012 shortly after turning 17, lasting until summer 2014. He was an unused substitute for the Chelsea first-team at the age of 15, in their League Cup clash against Newcastle and travelled with them to the 2012 UEFA Champions League Final at the Allianz Arena, which Chelsea won. On 30 August 2013, Chalobah signed a new five-year contract with Chelsea, stating that he wished to stay at Chelsea for his entire career.

Chalobah signed on loan for Championship club Watford on 31 August 2012 until January 2013. He made his first-team debut for the club on 18 September 2012 against Brighton & Hove Albion, coming on as 73rd-minute substitute for Fernando Forestieri. He later made his first start, lasting the full 90 minutes in Watford’s 2–2 draw with Bristol City on 23 September. On 17 November, Chalobah scored his first goal for Watford in a 2–1 win over Wolverhampton Wanderers. He scored his second goal for Watford in a 1–1 draw against Burnley, on 15 December, with a fine volley from the edge of the box.

On 2 January 2013, Chalobah extended his loan at Watford until the end of the 2012–13 season. In April, he scored with a 35-yard shot that Leicester City goalkeeper Kasper Schmeichel said he never saw; Watford won 2–1.

On 19 September 2013 Chalobah signed a loan deal with Nottingham Forest until 15 January 2014. Manager Billy Davies remarked upon his arrival, “He’s a tremendous young talent.” Chalobah made his debut from the start in Forest’s trip to Doncaster Rovers on 21 September 2013. On 5 October 2013, Chalobah was sent off after a second booking in the 67th minute after already picking up a yellow in the first half during the away match against Brighton & Hove Albion. Though Forest were down to 10 men, they saw out a 3–1 victory. Chalobah scored his first Forest goal in a 3–1 defeat away to Yeovil Town on 26 October 2013.

On 16 January 2014, Chalobah joined Middlesbrough on loan for the remainder of the 2013–14 season. On 25 January 2014, Chalobah made his debut against Leicester City, coming off the bench in the 77th minute to replace Emmanuel Ledesma. The match ended in a 2–0 loss for Middlesbrough. Chalobah made his first start on 1 February 2014, in a match against Doncaster Rovers which ended in a 0–0 draw. On 5 April 2014, Chalobah scored his first Boro goal in a 1–0 win at home to Derby County.

On 1 September 2014, Chalobah joined Premier League newcomers Burnley on loan from Chelsea until 2 January 2015. On 28 September 2014, Chalobah made his debut as a first-half substitute against West Bromwich Albion, although he could not prevent Burnley from losing 4–0. On 30 September, Chalobah suffered an injury after being hit with an elbow in the throat in a Burnley U21s match, which caused a cartilage to fracture. Chalobah returned to the pitch for the first team on 18 October 2014 against West Ham United. On 1 November 2014, Chalobah played his last match for Burnley against Arsenal, which ended in a 3–0 loss.

Chalobah was unable to break into the first team at Burnley and returned to Chelsea when the loan ended on 2 January 2015. During his time with Burnley, Chalobah only made four substitute appearances.

On 22 January 2015, Chalobah joined Championship club Reading on loan for the remainder of the season. On 24 January 2015, Chalobah made his debut in a FA Cup match against Cardiff City, as the team won 2–1 to advance into the fifth round. Three days later, Chalobah made his league debut against Millwall.

On 31 January 2015, in his home debut, Chalobah scored the second of a 2–0 victory over Sheffield Wednesday, helping the Royals to their first league win since December. During his time with Reading, Chalobah made a total of 20 appearances and started in every appearance.

On 1 September 2015, Italian club Napoli confirmed that Chalobah had joined the club on a season long loan from Chelsea. On 1 October 2015, Chalobah made his Napoli debut in a UEFA Europa League tie against Legia Warsaw. The fixture resulted in a 2–0 victory for the Italian team, with Chalobah featuring for five minutes after replacing Allan. On 26 November 2015, Chalobah was given his first Napoli start in a 1–0 victory over Club Brugge in the UEFA Europa League group stage tie. On 10 December 2015, Chalobah scored his first Napoli goal in a 5–2 win over Legia Warsaw, scoring Napoli’s first. On 6 January 2016, Chalobah finally made his Serie A debut in a 2–1 victory against Torino, replacing Marek Hamšík in the 89th minute.

In the summer of 2016, Chelsea rejected loan offers for Chalobah after new manager Antonio Conte had been impressed with Chalobah in pre-season, deciding that he should remain as part of Chelsea’s first-team group. Chalobah made his first-team debut on 20 September 2016, when he came off the bench to replace Michy Batshuayi in the 79th minute of a 4–2 win over Leicester City to advance to the Fourth round of the EFL Cup. On 15 October 2016, on his second league appearance for Chelsea, Chalobah provided a back-heel pass to play Victor Moses through for the third goal in a 3–0 victory over Leicester City.

On 13 July 2017, Chalobah signed for Premier League club Watford on a five-year contract for an undisclosed fee. On 26 September 2017, Chalobah was reported to have suffered a knee injury in a training session and was expected to have surgery.

In August 2017, Chalobah was named in the England squad for the first time for the 2018 FIFA World Cup qualifiers against Malta and Slovakia. He made his debut on 15 October 2018 as a substitute for Harry Winks in the first minute of stoppage time in England’s 3–2 win away to Spain in the 2018–19 UEFA Nations League A, with the competitive status of the fixture tying him to England permanently from then on. As he came on in added time for his only England cap, Chalobah officially has an England career of zero minutes, but in actual time he played for 6 minutes and 54 seconds. This is one second more than Martin Kelly, who holds the record for shortest England career in actual time played.