Jong Ajax Flemming played in the youth departments of ZSGOWMS, RKSV Pancratius and
Ajax. He made his professional debut for
Jong Ajax on 1 September 2017 in a game against
Jong PSV and scored on his debut. Flemming made 25 appearances during the 2017–18 season and won the Eerste Divisie title with Jong Ajax.
PEC Zwolle On 2 May 2018, Flemming signed a three-year contract with
PEC Zwolle, starting from 1 July 2018. There he made 28 appearances and six goals in his first season.
Loan to NEC On 2 September 2019, Flemming was sent on loan to
NEC for the
2019–20 season, who were competing in the second-tier Eerste Divisie. On 13 September, he made his debut in the 2–2 away draw against
FC Den Bosch. He scored his first two goals for NEC on 27 September, when the club drew 3–3 against
Go Ahead Eagles. With 13 goals, he became club top scorer of NEC for the 2019–20 season. As the season ended, he returned to PEC Zwolle.
Fortuna Sittard On 26 August 2020, Flemming signed a four-year contract with
Eredivisie club
Fortuna Sittard. He made his debut on 12 September in a 2–0 away loss to
Twente. On 26 September, he scored his first two goals in a 3–3 home draw against
AZ, one of them coming deep into injury time to secure the one point for Fortuna. He finished the season with 35 appearances, in which he scored 25 goals,
Millwall On 25 June 2022, Flemming joined
Championship club
Millwall on a three-year contract for a fee of £1.7 million rising to £2.5 million. He made his competitive debut for the club on 30 July, the opening day of the
season, replacing
George Honeyman in the 76th minute of a 2–0 home victory against
Stoke City. On 17 September, he netted his debut goal for
the Lions, breaking the deadlock in the 14th minute with a header from
Scott Malone's cross. The ball deflected off keeper
Daniel Grimshaw and scored his team's second in a 2–1 victory against
Blackpool. He secured his second goal on 5 October during a match against
Rotherham United from a long-distance strike. Just three days later, he achieved his first
brace for Millwall, propelling them to a 2–0 victory against
Middlesbrough at
The Den. He continued his impressive form the following week, on 15 October, by netting the decisive goal that secured a 2–1 away win against
Bristol City. This marked Flemming's fifth goal in five consecutive games. On 12 November, Flemming scored his first
hat-trick for the club, once again proving decisive in a 4–2 away victory against
Preston North End. He concluded the season with 15 goals in 44 appearances across various competitions, securing the position of the team's second-highest goalscorer, behind only
Tom Bradshaw. In recognition of his contributions, Flemming was honoured as the
Millwall Player of the Season in May 2023.
Burnley On 31 August 2024,
deadline day, Flemming joined
EFL Championship side
Burnley on a one-year loan deal with an obligation to buy at the end of the season, for a fee of £7million. He made his debut for the club on 14 September 2024, in a 1–0 win against
Leeds United. He scored his first goal for the club on 23 October 2024, in a 1–1 draw with
Hull City. On 20 May 2025, Burnley announced the player would be joining the club on a permanent basis. ==Career statistics==