While quarantine has become a widely used control strategy during the outbreak of the 2019 novel coronavirus disease (COVID-19), empirical research on whether and to what extent quarantine and attitudes towards COVID-19 affect mental health is scant. Using a cross-sectional online survey, this paper is the first from the Chinese outbreak to investigate how quarantine experiences and attitudes towards COVID-19 are related to mental health, and how these relationships change across the distribution of mental health scores. Using quantile regression analysis, we found that home self-quarantine is associated with a decrease in depression and an increase in happiness, while community-level quarantine is associated with decreased happiness, especially for those in the lower happiness quantile. We also found that favorable attitudes towards COVID-19 regarding the credibility of real-time updates and confidence in the epidemic control are associated with lower levels of depression and higher levels of happiness. These effects are stronger in the upper quantile of depression and the median quantile of happiness.