Koh taught at Universiti Sains Malaysia (USM), Penang from 1975 and rose to become Deputy Dean of Education in 1978.

[2] In the 1990 general election, Chief Minister Lim Chong Eu lost his state seat.

Koh was said to be pressured into leaving his position as Chief Minister to join federal politics.

[5] He eventually lost the parliamentary contest, while the opposition pact led by the Democratic Action Party (DAP) won the state of Penang.

Koh remained visible in the political scene after the election, and even participated in a televised public debate with his successor as Chief Minister, Lim Guan Eng.

This paved the way for Prime Minister Najib Razak to name Koh in his inaugural Cabinet.

His deputy, Chang Ko Youn, would take over as acting president until the party elections on 26 October 2013.