Koo is one of the three most successful South Korean players with careers in the German Bundesliga, alongside Cha Bum-kun and Son Heung-min.

Leading Boin High School to a runner-up place with an outstanding performance, he grabbed the attention of Jung Hae-seong, who was then the manager of Jeju United.

On 30 January 2011, Koo successfully moved to VfL Wolfsburg during the winter transfer window, signing a three-and-a-half-year contract for an undisclosed fee.

[6] On 12 February 2011, Koo made his Wolfsburg debut against Hamburger SV, coming on as a substitute in the 64th minute in the 1–0 home defeat.

[13] On 7 April, he scored the equaliser through Manuel Neuer's legs, but his goal couldn't prevent a 2–1 loss to Bayern Munich.

[15] During his loan period, he scored five goals and provided two assists in 15 appearances, becoming Augsburg's top goalscorer, although he played only half a season for them.

[17] In his second season at Augsburg, Koo was plagued by injuries, but he still helped his club survive relegation in the limited games he played in.

[23] On 31 July 2014, he contributed to a 1–0 victory by assisting the winning goal against Asteras Tripoli in the third qualifying round of the 2014–15 UEFA Europa League.

[24] On 7 August, he scored his first Europa League goal, but Mainz 05 were eliminated from the tournament after losing the second leg 3–1 to Asteras Tripoli.

[28] On 23 September, he won a penalty against Granit Xhaka, but it was insufficient to reverse Augsburg's defeat to Borussia Mönchengladbach.

[38][39] He wanted to contract with one of the top teams of the Bundesliga,[40] but finally joined Qatar Stars League side Al-Gharafa in August 2019.

He appeared in every game in the tournament, and scored the second goal during the second half of the bronze medal match against Japan, securing a 2–0 victory for Korea.

On 25 January 2019, Koo announced his retirement from international duty after South Korea lost in the quarter-finals of the 2019 AFC Asian Cup.

[52] On 24 June 2013, Koo married a Jeju woman three years his senior at the Sheraton Grande Walkerhill Hotel.