Tan is the Member of parliament of Malaysia for the Cheras constituency in Federal Territory of Kuala Lumpur.
He is also the National Chairman of the Democratic Action Party (DAP) in the governing Pakatan Harapan (PH) coalition.
Tan was elected to Parliament in the 1995 Malaysian general election, and is serving his sixth consecutive term.
It has been reported that Tan is known for his "fearless approach" in dealing with the government and "no-nonsense straight-up style".