Keizo Takemi was first elected to a member of the House of Councillors on 23 July 1995. He served until July 2007, and was Vice Minister of Ministry of Health, Labour and Welfare of the Abe Cabinet from September 2006 until August 2007. Takemi narrowly lost his seat in 2007. In 2012, he returned to the House of Councillors, and he won reelection in 2013 and 2019.
In 2006, United Nations Secretary-General Kofi Annan appointed Takemi to a High-level Panel on United Nations Systemwide Coherence, which was set up to explore how the United Nations system could work more coherently and effectively across the world in the areas of development, humanitarian assistance and the environment.
In March 2016, Takemi was appointed by United Nations Secretary-General Ban Ki-moon to the High-Level Commission on Health Employment and Economic Growth, which was co-chaired by presidents François Hollande of France and Jacob Zuma of South Africa.
In June 2019, World Health Organization (WHO) Director General Tedros Adhanom Ghebreyesus appointed him as WHO Goodwill Ambassador for Universal Health Coverage.