Heart of Midlothian King began his career with
Heart of Midlothian. He was a member of the club's
under-20 squad when he was promoted to the first team for their match against
Ross County on 27 October 2012, He remained an unused substitute on three further occasions made his first team debut playing from the start in a 1–0 victory over
Dundee in the
Scottish Premier League. He made a total of 8 appearances over his first season in senior football. His brother
Adam King broke through to the first team and made his debut on 26 December 2013 against
Kilmarnock but the two didn't feature together as Adam was substituted off at half time, the same time as Billy was substituted on. King's first career goal came on 30 March 2014 in the
Edinburgh derby as he came off the bench for
Dale Carrick in the 83rd minute before sealing a 2–0 win over
Hibernian with a 93rd minute goal. King made 34 appearances in all competitions over the
2013–14 season, scoring 3 goals in a season that saw the club enter administration which resulted in a 15 points deduction that contributed to their relegation to the
Scottish Championship for the first time since 1981. Following the club's return to the
Scottish Premiership, King found his game time reduced, starting just 11 of his 20 appearances up to January 2016. He made his first appearance for
Rangers against
Falkirk on 30 January 2016, coming on as a substitute for
Kenny Miller, scoring the winning goal in the 90th minute. He played the full 90 minutes against
Dumbarton on 5 April 2016 as
Rangers clinched the
2015–16 Scottish Championship title, earning promotion to the
Scottish Premiership. On 10 April 2016 came off the bench in the
2016 Scottish Challenge Cup Final at
Hampden Park as his side defeated
Peterhead 4–0 to win the trophy. He missed out on the
2016 Scottish Cup Final which his side lost to
Hibernian, as he was cup tied, having made 1 appearance for his parent club
Heart of Midlothian in an earlier round of the competition before joining
Rangers. King returned to Hearts at the end of his loan spell, having made a total of 13 appearances in all competitions for the club, scoring once. This turned out to be the last of his 96 appearances for Hearts as he moved on a season-long loan to
Inverness Caledonian Thistle two weeks later. He made his debut on 16 July 2016 away to
Cowdenbeath in the
Scottish League Cup. His first goal for the club came 10 days later in the same competition in a 5–1 win away to
Dunfermline Athletic at
East End Park. On 18 September 2016 he scored his first league goal for Inverness against
Scottish Premiership champions
Celtic in a 2–2 draw at the
Caledonian Stadium. King scored 3 goals in 32 appearances in all competitions over his loan spell at the club. He made his debut on 15 July 2017 in a
Scottish League Cup win over
Raith Rovers at
Tannadice Park. 4 days later he scored his first goal for the club and also got an assist in a 3–0 win away to
Buckie Thistle in the
Scottish League Cup. King was a key player for United over the season, scoring 8 goals in 48 appearances in all competitions over the season as they finished in third place. He made his debut for the club on 2 February 2019, coming off the bench for
Connor Ogilvie in a 1–1 draw with
Coventry City. King missed a penalty in the
Kent Senior Cup Semi Final penalty shootout loss against
Maidstone United on 19 March 2019. The final of his 4 appearances in all competitions for the club came in a 2–0 defeat at
Peterborough United on 6 April 2019.
Greenock Morton On 19 September 2019, King joined
Scottish Championship club
Greenock Morton until January 2020. His debut for the club came on 28 September 2019 in bad circumstances, as he came off the bench in the 77th minute in a 6–0 defeat against his old club
Dundee United at
Tannadice Park. He was substituted off in a 1–1 draw in a
Scottish FA Cup tie with
Brora Rangers in what turned out to be the final of his seven appearances for the club. It was confirmed on 30 December that he had left the club.
St Patrick's Athletic 2020 season On 8 January 2020 King signed for
League of Ireland Premier Division side
St Patrick's Athletic. He started pre-season off well, scoring a
hat-trick on his first start for the club in a friendly against Fermoy as his side won 6–0. He was voted as St Patrick's Athletic Player of the Month by the club's fans for the month of February, receiving his award before the home game against
Cork City on 6 March 2020, a game in which he scored his first competitive goal for the club in a 1–0 win. He played in 14 of the club's 18 league games over the course of the season which was shortened due to the
Coronavirus pandemic, as his side missed out on European football on the final day of the season, finishing in 6th place.
2021 season King scored his first goal of the
2021 season on 27 March 2021, coming off the bench for
Robbie Benson in the 80th minute before volleying home the winner in the 91st minute against newly promoted
Drogheda United in his side's first home game of the season. King's second goal of the season came on 23 April 2021 when he received a pass on the turn before beating 4 players and smashing his shot in from 20 yards to open the scoring in a 2–0 win away to
Finn Harps. On 24 May 2021, he scored an 84th minute header to level the game at 2–2 away to
Derry City at the
Ryan McBride Brandywell Stadium. King scored his first brace for the club on 23 July 2021, scoring the first and last goals of a 6–0 win over
Bray Wanderers in the
FAI Cup. On 3 September 2021, he scored the opening goal of a 3–2 win over
Longford Town, finding the top corner from 25 yards out. King opened the scoring in a 3–1
FAI Cup Semi-final win over
Dundalk on 22 October 2021 with a 25 yard strike in the 26th minute helping to earn a place in the
2021 FAI Cup Final. 3 days later, he strangely scored yet another opening goal in the 26th minute, again from 25 yards and against the same opposition in
Dundalk, with King's goal enough to earn his side the 3 points in the league fixture. He continued his rich vein of form by opening the scoring in the
Dublin City derby against
Bohemians on 1 November 2021. On 12 November 2021, he scored his 10th goal of the season by opening the scoring in a 2–2 draw with
Finn Harps that secured a second place finish in the league for his side. On 28 November 2021 King scored his penalty in the
2021 FAI Cup Final penalty shootout, as his side defeated rivals
Bohemians 4–3 on penalties following a 1–1 draw after extra time in front of a record
FAI Cup Final crowd of 37,126 at the
Aviva Stadium. King featured in all 41 of his sides games through the
2021 season, scoring 10 goals. On 11 February 2022, King was in the starting XI for the
2022 President of Ireland's Cup against
Shamrock Rovers at
Tallaght Stadium, as his side lost 5–4 on penalties after a 1–1 draw. On 6 May 2022, King scored his first goal of the season in a 4–0 win away to
Drogheda United, smashing home his side's third goal after providing the assist for
Darragh Burns' opener. On 24 June 2022, King scored a brace in a 2–2 draw away to
Finn Harps. He followed that up by scoring the opener in a 3–0 win over
Drogheda United at
Richmond Park. King's first appearance in European football with the club came on 28 July 2022 in a 0–0 draw away to
Slovenian side
NŠ Mura with his side winning 6–5 on penalties to earn a tie with
Bulgarian side
CSKA Sofia in the next round. He featured in both a 1–0 away win in Sofia and in the home leg which saw his side narrowly beaten 2–0 following a controversial late penalty. He finished the season with 4 goals in 34 appearances in all competitions as his side finished in 4th place in the league, securing European football once again. On 29 September 2024, King was part of the side that won the
2024 USL Cup by defeating
Forward Madison 5–4 on penalties after a 1–1 draw, to become the competition's first ever winners.
Arbroath On 25 January 2025, King returned home, signing for
Scottish League One club
Arbroath. He scored 1 goal in 7 appearances by the end of the season, as his side won the
2024–25 Scottish League One title to gain promotion to the
Scottish Championship.
Bonnyrigg Rose On 31 May 2025, King signed for
Bonnyrigg Rose, who had recently been relegated to the
Lowland League. ==International career==