Southampton 2017–2022 On 21 February 2017, Smallbone signed a professional contract with
Southampton. He made his professional debut for Southampton in a 2–0
FA Cup victory over
Huddersfield Town on 4 January 2020, scoring the opening goal. On 22 February 2020, Smallbone went on to make his
Premier League debut in a 2–0 victory over
Aston Villa. Having just started to appear regularly for the first team, on 16 January 2021, Smallbone suffered an
anterior cruciate ligament injury in a 2–0 defeat at
Leicester. As a result, he was expected to be out of action for up to six months. On 26 October 2021, Smallbone made his first professional appearance since his injury, replacing
Stuart Armstrong in Southampton's defeat to
Chelsea after a penalty shootout in the
EFL Cup.
2022–2025 On 23 July 2022, Smallbone joined
Stoke City on a season-long loan. He scored his first goal for Stoke in a 2–0 win at
Preston North End on 15 October 2022. Smallbone was a regular under
Alex Neil in
2022–23, making 46 appearances, scoring three goals as Stoke finished in 16th position. On 28 August 2023, Smallbone signed a three-year contract extension with Southampton. He scored his first league goal for the club on 30 September 2023 in a 3–1 victory against
Leeds United. On 17 May 2024, Smallbone scored the opening goal in a 3–1 victory against
West Bromwich Albion in the second leg of the
EFL Championship play-off semi-final, helping to secure his team's place in the
final. Later that month, on 26 May, he provided an assist for
Adam Armstrong in a 1–0 victory against Leeds United in the final, ensuring his club's promotion to the Premier League. During a 3–2 home defeat against Leicester City on 19 October 2024, Smallbone made his return from an injury after he came off the bench, but manager
Russell Martin feared he "tweaked" his hamstring during the game. Martin later revealed that Smallbone was expected to be sidelined for "a little bit of time" after he battled through the pain barrier to play against Leicester City. On 8 March 2025, he scored his first Premier League goal in a 3–1 defeat against
Liverpool.
2025–present On 1 September 2025, Smallbone joined
Millwall on a season-long loan. He made his first appearance for the club on 22 September in a 1–0 victory against
Watford after he replaced
Aidomo Emakhu in the 67th minute. ==International career==