He is a former Democratic Party member of the Legislative Council of Hong Kong representing the New Territories East geographical constituency.

The proposal draw unprecedented controversy in the pan-democratic camp as the leaders of the Democratic Party had met with and sought approval from the officials of the Central Government Liaison Office.

[citation needed] He subsequently decided to vote against the proposals, and announced in his Legco speech that he would quit the party because "small, but critical differences of opinion" prevented him from fulfilling his election pledge to strive for universal suffrage in 2012.

[2] Although once denied rejoining electoral politics, Cheng changed his mind and contested the 2016 legislative election in New Territories East.

[3] Cheng is married to Chan Kwai-ying, who is a cousin of his fellow Democrat Wong Sing-chi, and is a father of two.