•Compared with 2016 and 2017, the population stress level, prevalence of anxiety, and the depression symptoms drastically increased during the COVID-19 outbreak.•Older and less-educated respondents tended to have deteriorated mental health outcomes.•Public mental health interventions are urgently needed particularly for the vulnerable groups. To compare the mental health burden before and during the COVID-19 outbreak and identify the vulnerable groups by sociodemographic factors. We analyzed repeated cross-sectional data from the Hong Kong Family and Health Information Trend Survey (FHInTS) in 2016 (N = 4036) and 2017 (N = 4051) and the COVID-19 Health Information Survey (CoVHInS) in April 9–23, 2020 (N = 1501) using population-based random samples of general adults by landline telephone and online panel. Stress (Perceived Stress Scale 4), anxiety symptoms (General Anxiety Disorders 2), depression symptoms (Patient Health Questionnaire-2), subjective happiness (4-point Likert item), and sociodemographic factors were collected. Compared with 2016 and 2017, the stress level increased by 28.3%, prevalence of anxiety increased by 42.3%, and the depression symptoms and unhappiness have doubled (all P for trends <0.001) during the COVID-19 outbreak. The increases in stress levels were significantly larger among older and less educated respondents (P for interactions <0.001). Hong Kong had a mental health emergency even with no lockdown and well-managed outbreaks. Older and under-privileged people will suffer most. Public mental health interventions are urgently needed particularly for the older adults and individuals with primary or lower education attainment.