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Aged 16, Kingsley made his first-team debut for Falkirk on 12 April 2011 in a 2–1 win versus Partick Thistle in the Scottish First Division.[2] He sustained an injury in a 2011–12 Scottish League Cup tie against Brechin City that kept him out of action for over three months. He returned to the starting lineup in a League Cup win against Dundee United. In March 2013, Kingsley signed a new contract with Falkirk.[3] Kingsley made his 100th appearance for Falkirk on 5 April 2014, in a 5–0 victory over Cowdenbeath. He was also voted in to the SPFL Championship Team of the Year for season 2013–14.
Swansea City
On 30 June 2014, Kingsley signed for Swansea City on a three-year deal for an undisclosed fee.[4]
On 18 February 2015, Kingsley joined English League One side Yeovil Town on a month's emergency loan which was later extended until the end of the season.[5] Kingsley played in numerous positions in his 12 appearances for the Glovers including left-back, centre-back and midfield, and received praise for his versatility from Yeovil boss Terry Skiverton.[6] Yeovil were subsequently relegated to League Two at the end of the season and Kingsley returned to Swansea City.
Kingsley joined English League One side Crewe Alexandra on an emergency loan in August 2015.[7] He made 12 appearances for them before returning to Swansea after the emergency loan expired in November.
He made his Swansea City debut in their FA Cup 3–2 defeat away to Oxford United, on 10 January 2016.[8] He made his Premier League debut on 2 March 2016 in Swansea's 2–1 victory away to Arsenal.[9]
Hull City
Kingsley moved to Championship club Hull City in August 2017 for an undisclosed fee.[10] He was released by Hull at the end of the 2019–20 season.[11]
Heart of Midlothian
Kingsley signed a one-year contract with Hearts on 6 October 2020.[11] He played and scored for Hearts in the 2020 Scottish Cup Final, which finished 3–3 after extra time, but his effort in the subsequent penalty shootout was saved by Conor Hazard as Hearts lost 4–3 on penalties.[12]
On 21 January 2021, Kingsley signed an 18-month contract extension with Hearts, keeping him at the club until the end of the 2021–22 season. He further extended his Hearts stay in January 2022, signing a three-and-a-half-year contract and keeping him at the club until the summer of 2025.[13] He then further extended his contract on 6 August 2024 with a new three-year contract until the summer of 2027.[14]
In April 2012 Kingsley was called up to the Scotland under-18 squad for two games against Serbia in Belgrade on the 17th and 19th of the month. He played 90 minutes in the first game, which ended 1–1, and 60 minutes in the second game, where Scotland won 4–1. In season 2012–13 he appeared for Scotland under-19s against Germany and the Netherlands. He was selected for the squad for the 2014 UEFA European Under-19 Football Championship qualification matches in October, playing in the third game against Switzerland. This was followed in March with an appearance in the friendly match against Sweden. In May he played in two of the Elite qualifying round games in Belgium against Belgium and Georgia. In March 2015, Kingsley made his Scotland under-21 debut in a 2–1 win against Hungary. This was followed up with four appearances for the side against France twice, Northern Ireland and Iceland in their Euro Championship Qualifying group.
On 20 May 2016, Kingsley received his first call-up to the Scotland national football team for their friendlies against Italy and France.[18] He made his Scotland debut as a substitute in a 3–0 defeat against France, on 4 June 2016.[19]
Kingsley was recalled to the national team squad in September 2022 as a replacement for the injured Kieran Tierney and Scott McKenna.[20] He appeared as a substitute in a goalless draw with Ukraine that secured a promotion for Scotland in the UEFA Nations League.[21]