From 2016 to 2017, Au was the convenor of the Civil Human Rights Front. In the 2018 Legislative Council by-election triggered by oath-taking controversy, Au replaced Demosistō candidate Agnes Chow who was disqualified over her alleged pro-independence stance. He was elected with more than 130,000 votes. Au was a member of the Democratic Party until 2017. In December 2019, Chow's disqualification was ruled unlawful by a court, invalidating the by-election result and unseating Au.