In June 2005, Koo served on the
National Assembly. In September 2013, A primary held in May 2014, after the Sunflower Movement, was won by
Pasuya Yao, and Koo dropped out of the race. Yao later dropped out of the race, endorsing
Ko Wen-je, who won the
mayoral election as an independent candidate. Koo, then the director of the Judicial Reform Foundation, was selected for the Democratic Progressive Party's proportional representation ballot in November 2015. Listed fourth on the ballot during the
2016 legislative election, he won a seat in the
Legislative Yuan. In his time as legislator, he called for the establishment of a government commission on human rights. Koo also coauthored amendments to the Act Governing Relations with Hong Kong and Macau in an attempt to simplify the process for political asylum-seekers from those areas to Taiwan. He also proposed an amendment to the Narcotics Hazard Prevention Act, stating that people involved in the illegal drug trade should be treated for addiction prior to being put on trial. The Act Governing the Handling of Ill-gotten Properties by Political Parties and Their Affiliate Organizations, which he helped to write, was passed in July and Koo was named to a
commission set up to investigate questionable assets in August. He stepped down from the Legislative Yuan to take the appointment, and was succeeded in office by
Julian Kuo. Koo assumed the committee chairmanship despite the
Kuomintang citing Article 20 of the Act, which requires nonpartisan committee members, in its objections to Koo's leadership. Koo named most of the committee members on 24 August, and the group was officially established on 31 August. Koo was appointed to the chairmanship of the
Financial Supervisory Commission in September 2017, succeeding
Lee Ruey-tsang on the same day that
William Lai replaced
Lin Chuan as premier. Koo stated shortly before taking control of the FSC that he sought to implement a "differentiated management style" in which financial institutions that ranked higher would be allowed more regulatory freedom to innovate within the financial services sector, and those institutions that did less well would be granted less latitude. Koo left the Financial Supervisory Commission in May 2020 and became the secretary-general of the
National Security Council. On 17 August 2022, in the aftermath of then
Speaker of the United States House of Representatives Nancy Pelosi's
visit to Taiwan on 2–3 August, China blacklisted seven Taiwanese officials including Koo as "
diehard "Taiwan independence" separatists" due to their support for
Taiwanese independence. The blacklist bans them from entering
mainland China and the
Special Administrative Regions of
Hong Kong and
Macau, and restricts them from working with Chinese officials. Chinese state-run tabloid
Global Times labelled Koo and the six officials as "diehard secessionists". On 25 April 2024, Koo was appointed
Minister of National Defense in the incoming
William Lai presidential administration. Koo became the first civilian leader of the
Ministry of National Defense since
Andrew Yang. On 23 May, China conducted
military exercises around Taiwan. On 6 June, Koo replied to the inquiry of
MP Puma Shen in the Legislative Yuan with the military reform measures: • Cancelling the traditional
bayonet piercing format that does not conform to the practical melee combat principle. • Abolishing the formalistic mandatory daily reports during personnel's off-camp weekends and holidays. • Personnel are free to travel abroad individually without the restriction in groups only. • Removing the
goose marching shows in the
Military Academy, same as the
combined arms brigade troops back to their combat training duty. • Operational and tactical commands are authorized to the Chief of the general Staff as before for exercise efficiency. • Cancelling the order retaining either the chief or deputy commanders in camp all the time, to restore the regular holiday and vacation schedules. • Following the US military example to set the examination protocols of security clearance by personnel's actual working authorization instead of his/her rank. ==Personal life==