Background Previous research has demonstrated that the COVID-19 pandemic led to a global increase in mental distress. However, few studies have examined the impact of the pandemic on mental health stigma. Aims To investigate changes in measures of mental health stigma, including knowledge, attitudes and behavioural intentions, in 2021 and 2023 in Hong Kong; to examine the mediating role of attitudes on the relationship between knowledge and behavioural intentions; and to explore how disclosure of mental illness contributes to enhanced overall well-being. Method Data were collected as part of a larger research project focusing on mental well-being in Hong Kong. A total of 1010 and 1014 participants were surveyed in 2021 and 2023, respectively. The participants were Hong Kong residents aged 18 years and above. Results Our findings demonstrate that all measures of mental health stigma showed increases in severity between 2021 and 2023. In addition, our mediation analyses observed both full and partial mediation effects of attitudes on the relationship between knowledge and behavioural intentions. The results also showed that mental illness disclosure was associated with higher well-being; however, despite these benefits, there was a decrease in willingness to disclose in 2023 compared with 2021. Conclusions This study highlights the ongoing issue of mental health stigma in Hong Kong. Future mental health programmes and interventions should aim to address various facets of mental health knowledge, including symptom recognition, access to support resources and the deleterious consequences of mental health stigma.