Millwall King signed a one-year contract with
Millwall in August 2014, having left the
Crystal Palace youth team earlier in the season. He joined
National League club
Welling United on an initial one-month youth loan on 24 July 2015. He went on to make 20 appearances for the "Wings", before he was recalled by Millwall on 21 December. King returned to the National League on 19 February 2016, joining
Braintree Town for an initial 28-day emergency loan. In his second appearance for the club, against
Guiseley at
Nethermoor Park, King became involved in a controversial incident during an 1–1 draw. He conceded the equalising goal after
Oliver Norburn broke a fair play protocol and lobbed him from 30-yards out rather than passing the ball to him, after Braintree had kicked the ball into touch to allow an injured Braintree player to receive treatment; a five-minute fracas followed but the goal was allowed to stand after security staff separated the players and management, and Guiseley manager
Mark Bower refused to allow Braintree to walk through and score. Bower went on to say: "Their keeper stood there with his arms in the air and allowed the ball to into the net. It put us in a really difficult position whether we should allow them to score or not but we decided no. I think their keeper was trying to be clever and had simply let the ball go in." King said he was "bewildered" by Bower's statement, and speaking on the incident itself said that: "It is one of those things that will be remembered for a long time, I'll be remembered for a long time and it will carry on forever". The incident was described as "one of the biggest on-field controversies seen in non-league football", and Braintree manager
Danny Cowley said that: "it's the worst thing I've seen on a football pitch and it was disgraceful unsporting behaviour on their part". The following week, Guiseley chairman Phil Rogerson released a statement recognising the fair-play convention had not been followed: "... myself, Mark [Bower] and the club find the situation most regrettable and not in line with the general ethos of Guiseley AFC. Fair play is and always has been at the heart of the club. The decision to continue playing as normal after the goal was taken on the spur of the moment and under extreme pressure, not helped at all by the heated atmosphere." King made his first team debut for Millwall in a 2–1 defeat to
Nottingham Forest in an
EFL Cup Second Round match at
The Den on 23 August 2016. King signed an 18-month contract extension with Millwall on 4 January 2018. On the same day, he joined
League Two side
Stevenage on loan for the rest of the
2017–18 season. On 27 June 2018 King joined
AFC Wimbledon on a season long loan. King started the season as first choice goalkeeper but soon fell behind the Dons reserve goalkeeper
Joe McDonnell in the pecking order as the Dons struggled at the bottom of the table. His season-long loan was cut short when recalled by Millwall in January having started only 11 League games for the Dons, who had already replaced him with the loan of
Aaron Ramsdale on loan from
Bournemouth.
Newport County On 7 June 2019, King joined
League Two club
Newport County on a two-year contract. On 3 August 2019 he made his debut for Newport in a 2–2 draw against
Mansfield Town, saving a second half penalty. In September 2019, King was nominated for the August
PFA League Two Player of the Month Award, after five successive clean sheets, and more than 500 minutes of football played without conceding for the South Wales side, who had climbed to second place in the league. King scored the first goal of his career on 19 January 2021, with a wind-assisted goal kick in the 12th minute of Newport's 1–1 League Two draw at
Cheltenham Town. On 21 January 2021, his goal was confirmed to have broken the
Guinness World Record for longest football goal, with a distance of , a record previously held, since November 2013, by
Asmir Begović. King played for Newport in the
League Two playoff final at
Wembley Stadium on 31 May 2021 which Newport lost to
Morecambe, 1–0 after a 107th-minute penalty. On 4 June 2021 it was announced that he would leave Newport County at the end of the 2020–21 season, following the expiration of his contract.
Salford City On 7 July 2021,
Salford City confirmed that they had signed King on a two-year deal.
Northampton Town On 14 January 2023, King signed for fellow League Two club
Northampton Town on a six-month deal after agreeing a release from his Salford contract. Having been dropped for
Alex Cairns, King's availability arrived at a good moment for Northampton due to an injury to
Lee Burge.
Wolverhampton Wanderers On 3 July 2023, King joined
Wolverhampton Wanderers on a free transfer following his release from Northampton Town. He featured as a 78th minute substitute in a friendly against
Stade Rennais in Wolves' 3–1 win. This marked his debut for the club. In February 2024 King signed a two-year contract extension with Wolves. King made his
Premier League debut for Wolves at the age of 30 in the 1–1 draw against
Brentford on 25 May 2025 as a second half substitute.
Everton On 15 August 2025 King joined Everton on a two year contract. ==International career==