The coronavirus disease 2019 (COVID-19) pandemic has had a profound impact on individuals’ mental health and well-being worldwide. This study investigated the prevalences of and association between mental health and quality of life (QOL) in Thailand after the COVID-19 outbreak. A cross-sectional study was conducted among Thai individuals aged ≥ 20 years across all regions. Descriptive statistics, chi-square tests, and multiple logistic regression analyses were performed to examine the association between mental health and QOL. A total of 1,133 participants (mean age: 35.1 ± 17.2 years) completed the survey. The prevalence of depression with PHQ-9 instrument was 19.4%. Depression was significantly associated with those who have had family members die from COVID-19 infection (adjusted odds ratio [AOR] 2.27, 95% confidence interval [CI] [1.13–4.52]). The percentages of depression, anxiety, and stress with DASS-21 instrument were 32.4%, 45.4%, and 24.1%, respectively. Smokers and alcohol consumption had approximately 1.5-time higher risk of stress compared with non-smokers and no alcohol consumption (AOR = 1.50, 95% CI [1.01–2.24], AOR = 1.48, 95% CI [1.09–2.02], respectively). An association was observed between socioeconomic factors such as job and income loss and mental health outcomes. Depression, anxiety, and stress were significantly negatively associated with QOL. This study demonstrates a strong association between mental health and QOL among Thai people after the COVID-19 outbreak. The findings underscore the need for interventions targeting lifestyles, including those addressing alcohol consumption and smoking, especially among those who have had family members die from COVID-19 infection and mental health support services that can address depression, anxiety, and stress to improve the overall well-being of the population.