Brighton & Hove Albion Development side Molumby became a key part of the Brighton youth teams, playing for both the under-23s and the under-18s. He also played in the semi-final of the
Sussex Senior Challenge Cup.
2017–18 Molumby made his debut for the
Seagulls first team on 22 August in an
EFL Cup second round match against
Barnet, starting the move that led to the only goal of the game. He made his second appearance for the senior side in the next round of the EFL Cup, playing 120 minutes against
Bournemouth alongside fellow Irishman
Dessie Hutchinson in the eventual away loss. On 13 August Molumby made his Millwall debut in the first round of the
EFL Cup where he started in the 2–1 away win over
West Brom. He featured in a
Premier League match day squad for the first time on 26 September where he remained an unused substitute in the 3–2 home loss against
Manchester United. Four days later Brighton faced
United again this time in the EFL Cup where Molumby started and played 81 minutes of the 3–0 home defeat falling short of a quarter final place. Molumby made his Premier League debut on 21 November, coming on as 90+5th-minute substitute helping the
Seagulls hold on to their lead over
Aston Villa where it finished 2–1 away from home.
Preston (loan) On 5 January 2021, Molumby joined
Championship side
Preston North End on loan until the end of the season. He made his debut four days later, starting and playing 75 minutes of the eventual 4–1 third round
FA Cup defeat away at
Wycombe. A week later he made his league debut for
The Lilywhites coming on as a 79th-minute substitute in a 2–0 away loss against
Bristol City. On his third appearance he assisted a
Scott Sinclair goal in a 1–0 away win over
Birmingham City on 20 January.
West Bromwich Albion Initial loan On Friday 27 August 2021, Molumby signed with
EFL Championship club
West Bromwich Albion on loan until the end of the season, with a view to a permanent transfer. He made his debut on 28 September, coming on as an 81st-minute substitute replacing captain
Jake Livermore in the 4–0 away win over
Cardiff City. Molumby made his first start on his fourth appearance on 20 October, playing 79 minutes of the eventual 2–1 away defeat at
Swansea City. Three days later, he made his second start for
The Baggies where he played the whole match of the 3–0 home win over
Bristol City. On 26 November, Molumby was sent off for the first time in his career for a second bookable offence in the 0–0 home draw against
Nottingham Forest. He scored his first goal for
The Baggies on 22 February 2022, his second career goal, putting
Albion in front, however
Middlesbrough fought back and won 2–1 at the
Riverside Stadium.
Permanent signing On 4 May 2022, Molumby signed for the
Baggies on a permanent deal agreeing a three-year contract with the club with the deal officially going through on 1 July. He scored his first goal of
the season on 14 January 2023, smashing home the equaliser after coming from 2–0 down before going on to win 3–2 away at
Luton Town. On 15 April, Molumby scored his first career brace, scoring the equaliser and then the winner within 10 minutes in the 2–1 away win over
Stoke City. == International career ==