*Club domestic league appearances and goals, correct as of 23:33, 23 December 2022 (UTC) ‡ National team caps and goals, correct as of 19 November 2014
A member of the club's under-20 squad,[5] King was predominantly a midfielder, however could also play right back[6] and centre back.[7] Having yet to feature for the first team in September 2013, with the transfer window coming to a close, Premier League outfit Swansea City made an offer believed to be a five-figure sum of money for King.[8] However, Hearts administrators BDO rejected the offer.[9] Having been included in the match day squad for every game so far that season,[6] yet not featured, he made his first team debut for Heart of Midlothian (Hearts)[10] as a substitute in a friendly against German Bundesliga side Wolfsburg[11] on 13 November 2013.[12] He made his full professional debut in a Scottish Cup tie against Celtic on 1 December 2013, coming on as a substitute in the 79th minute replacing Jamie Hamill.[13] He went on to make his first Scottish Premiership appearance on 21 December, playing from the start against Celtic at Celtic Park.[14]
In January, Swansea renewed their interest in King and a further bid was accepted by the club.[15] In all he made three appearances for Hearts.[16] He has featured at both under-18[2] and under-19 levels for Scotland.[17] King was nominated by STV Sport as one of 14 young players in Scottish football to watch in 2014.[4]
In July 2015, King joined League One team Crewe Alexandra on loan until 6 January 2016.[23] King returned to Swansea on 5 January 2016. On 5 August of the same year he signed a season-long loan deal with League One side Southend United,[24] but this was cut short in January 2017, with the player returning to Swansea City.[25] On 29 January 2018 King signed on loan for League Two side Mansfield Town for the rest of the season.[26] On 2 July 2018, he joined Peterborough United on another loan deal set to last until January 2019,[27] but he returned to Swansea City the following month after his loan was terminated because of injury.
Dundee United
King left Swansea to join Scottish Championship club Dundee United on a three-year contract in July 2019. His brother Billy had left United the previous season.[28] In February 2020, he joined Queen's Park of Scottish League Two on loan for the rest of the 2019–20 season.[29] On 29 January 2021, King joined Raith Rovers on loan until the end of the season.[30]
Alloa Athletic
King was released by Dundee United at the end of the 2020/21 season, he went on trial at Falkirk but was offered an immediate deal by Alloa Athletic and instead accepted the guaranteed deal and signed for The Wasps in June 2021.[31]