On 20 May 2015, Ki was voted Swansea City's Player of the Year as he scored eight goals in 33 appearances, with whom he also won the EFL Cup in 2013.

[7][8] In 2001, he went to Brisbane, Australia, to study at John Paul College under the BSP (Brain Soccer Program) overseen by Jeff Hopkins.

[7][10] Ki returned to South Korea and joined FC Seoul, where he played alongside national teammate Lee Chung-yong.

On 29 October 2008, Ki scored the winning goal against Seoul's biggest rivals Suwon Samsung Bluewings in the 92nd minute.

[12][13] On 25 August 2009, it was revealed that contact between Celtic and FC Seoul had occurred regarding the possibility of Ki's transfer to the Parkhead club.

[23] On 26 December 2010, Ki scored against St Johnstone in a 2–0 victory in the SPL,[24] in what was his last match for Celtic before travelling to Qatar to play for South Korea in the 2011 AFC Asian Cup in January.

Scoring the second goal in a 2–0 victory, a 25-yard left foot strike into the right hand bottom corner and also won the Man of the Match award from the official Celtic website.

Over the course of the 2011–12 campaign, Ki scored seven goals and had six assists, playing a key role in helping Celtic win the Scottish title.

Although he failed to replicate the scoring form he displayed at Celtic,[37] his passing style drew praise from supporters and critics,[38] and he finished the season with 38 appearances in all competitions.

[39][40] The tactical switch proved to be a success, as Swansea ran out comfortable 5–0 winners against Bradford City[39] as Ki won his first trophy with the Welsh club.

Everton goalkeeper Tim Howard played a short pass to Leon Osman who was robbed of the ball by Ki.

[53] Ki became a regular in the team under new manager Garry Monk, and by the end of December, had started in every Premier League fixture.

[59] He made three less appearances in the 2016–17 Premier League where, which the club struggled in for the majority of the season, Swansea narrowly avoided relegation,[60] but the player did not score in that campaign.

[65] On 29 June 2018, Ki signed a two-year deal with Newcastle United after the expiration of his Swansea City contract, thus reuniting with former midfield partner Jonjo Shelvey.

[66] Manager Rafa Benitez gave Ki his first start for Newcastle on 26 August against Chelsea and the following weekend against Manchester City, both resulting in 2–1 defeats.

[69] His good form was halted as the South Korean had to join his national team to take part in the AFC Asian Cup in the UAE.

Despite Newcastle securing safety in the Premier League, manager Rafa Benitez refused to extend his contract and was replaced by Steve Bruce.

Up until January, Ki would rarely feature on the team sheet, moreover illness and a knock sustained in training in December furthered hindered his chances to play.

Bruce and Newcastle allowed Ki to ponder his options in search of more game time during the winter transfer window, and on 31 January 2020, he left the club by mutual consent.

[75] On 25 February 2020, Ki joined RCD Mallorca for the remaining of the season, with the Balearic club engaged in a relegation battle from the Spanish top tier.

[76] He made his La Liga debut on 7 March, replacing Takefusa Kubo in the last fifteen minutes of a vital 2–1 away win against Eibar, thus becoming the first Korean player to wear the shirt of Los Bermellones.

[81] Caretaker manager Kim Ho-young would then give Ki time to regain fitness by bringing him off the bench the three following games, and his impact on the team was quickly praised for providing more stability in FC Seoul's midfield.

He scored decisive goals in three consecutive games which Seoul won by a single-goal margin, including a last-minute winner against Incheon United on 13 March.

Ki also played for South Korea in the 2015 Asian Cup during January, helping his country reach the final against hosts Australia.

The club, who faced financial difficulties, managed to climb to the country's top tier under his tenure but failed to establish itself in the K League.

[112] On 25 January 2011, during the 2011 AFC Asian Cup semi-final match between Japan and South Korea, Ki scored the opening goal through a penalty kick.

[113] Ki created a controversy by celebrating his goal by scratching his cheek in front of a pitch-side camera in an allegedly racist slur at Japanese people.

[114] In the summer of 2013, it was widely reported Ki had previously insulted then-South Korea national team manager Choi Kang-hee on his Facebook page before and after the match against Kuwait in February 2012.

Although the press release detailing the accusations did not name Ki as a perpetrator, it described a "star player who recently joined a club in the greater Seoul area", and the acts took place while in sixth grade of elementary school at South Jeolla Province in the year 2000.

Ki, first via his agency C2 Global and then on social media, categorically denied the accusations stating "he had no connection whatsoever" to the case and threatened to take legal action.