Sydney FC King made his professional debut for
Sydney FC in a Round 27 clash against
Newcastle Jets, playing the full game in a 2–0 loss at
McDonald Jones Stadium. On 9 May 2019, he signed his first professional contract with the club, penning a one-year deal for the 2019–2020 season. King cemented his spot in the Sydney team, and started in every game for Sydney during the
2020-21 season with regular fullback
Michael Zullo missing the majority of the season with a recurring calf injury. His consistent performances during the season resulted in him winning the
2020–21 Young Footballer of the Year at the end of the season, becoming the first Sydney FC player to do so.
Odense Boldklub On 29 January 2022, it was announced that King had signed with Danish club
OB after triggering a six figure release clause.
Loan to Sydney FC In February 2023, King was loaned back to former club
Sydney FC until the end of the
2022–23 A-League Men season. King made 13 appearances in his loan spell, including starts in an elimination final win over
derby rivals
Western Sydney Wanderers, and in both legs of the semi-final tie against
Melbourne City.
Return to Sydney FC In June 2023, King re-signed with Sydney FC on a permanent basis, signing a three-year deal with the club. King made his 100th appearance for the club on 29 December 2023, in a 3–1 win over
Wellington Phoenix at
Allianz Stadium. He scored his first goal for Sydney on 30 March 2024, in a 2–0 win over the
Central Coast Mariners. On 23 August 2025, King made his 150th appearance for Sydney FC against
Auckland FC in the quarter-finals of the
2025 Australia Cup. ==International career==