In the 2020 Legislative Council election, Cheng was nominated by the Civic Party to run in the Hong Kong Island constituency. He replaced incumbent Tanya Chan, who was barred from running due to her suspended sentence over the 2014 Occupy protests. In the pro-democracy primaries in July 2020, Cheng took fourth place, after Ted Hui, Tiffany Yuen, and Fergus Leung. His 11,090 votes (12.26% of the electorate) secured him a nomination spot in the legislative election.