In 2017, after a legislative seat in West Kowloon was vacated following the oath-taking controversy, veteran lawmaker and former chairman Frederick Fung sought for nominations from ADPL to contest the March 2018 Kowloon West by-election. However, Mok insisted the party should nominate a younger member instead. She left the ADPL on 26 June 2017 after the party resolved to nominate Fung. She later joined the Democratic Party. In 2019, she fought the Wong Tai Sin District Council seat vacated by party chairman Wu Chi-wai, and retained the seat.