1984 general election

Mah entered politics in the 1984 general election when he contested as a candidate of the governing People's Action Party (PAP) in Potong Pasir Single Member Constituency (SMC) against Chiam See Tong of the opposition Singapore Democratic Party. He lost to Chiam, who won with 60.28% of the vote. Lee Kuan Yew, the PAP's leader, was thought to have played a role in Mah's electoral defeat, having been seen as being unnecessarily dismissive in comparing Chiam's average GCE Ordinary Level results to Mah's stellar academic achievements.^[3]

1988–2015

During the 1988 general election, Mah joined a three-member PAP team contesting in Tampines Group Representation Constituency (GRC) and they won with 61% of the vote, so Mah became a Member of Parliament representing the Tampines East ward of Tampines GRC. Since then, he and the PAP team in Tampines GRC had won the subsequent five general elections (1991, 1997, 2001, 2006 and 2011) against the opposition Singapore Democratic Alliance and National Solidarity Party. Excluding an uncontested walkover in 1997, Mah and the PAP team in Tampines GRC won with results of between 73.34% (in 2001) and 57.22% (in 2011) of the vote during those five general elections.

In 1990, Mah was appointed Minister of State at the Ministry of Trade and Industry and Ministry of Communications. A year later, he was appointed to the Cabinet as Acting Minister for Communications while concurrently holding the post of Minister of State for Trade and Industry.

Mah served as Singapore's Minister for Communications from 1991 to 1999. He concurrently served as the Minister for the Environment from 1993 to 1995. He was appointed Minister for National Development in 1999 and served until 2011. He also served as Deputy Leader of the House in Parliament from 2002 to 2007, and Leader of the House from 2007 to 2011.

On 20 May 2011, Mah said at a Building and Construction Authority event that he accepted responsibility over public unhappiness due to a sharp increase in housing prices over the previous years.^[4] He stepped down from his Cabinet position of Minister for National Development after the 2011 general election.^[5] In a 2013 Chinese-language book published by the Hainan Culture and Heritage Centre, Mah refuted rumours that he had to step down because he had seemingly been blamed for growing public unhappiness over sky-rocketing property prices, claiming that he stepped down to make way for younger, talented people to take over.^[6]

Throughout his political career, Mah had also held other positions, including chairman of the board of directors of NTUC Comfort from 1983 to 1986; chairman of the National Productivity Board, National Productivity Council and Skills Redevelopment Fund Advisory Council; chairman of the board of governors of the Singapore Institute of Labour Studies from 1990 to 2002; and president of the Football Association of Singapore from 1999 to 2004.^[2]

Mah announced his retirement from politics on 23 August 2015 just before the 2015 general election.^[7]