Although studies have extensively discussed the effects of COVID-19 on global health behaviors, little is known about the extent of COVID-19 preventive behaviors and their negative effects on the working-age population of Bangkok, Thailand. Therefore, this study aimed to examine health-risk behaviors, COVID-19 preventive behaviors, and the impact of the COVID-19 pandemic among the Thai working-age population. An online cross-sectional survey was conducted among working-age adults through a self-administered questionnaire. Data were analyzed with descriptive statistics and stepwise multiple regression. Of the 313 Thai participants (227 females, 72.5%), 84.0% had high levels of health-risk behaviors for preventing COVID-19 infection, most respondents (89.1%) had high levels of knowledge about COVID-19, and most respondents (61.7%) had high levels of overall COVID-19 preventive behaviors. Stepwise multiple regression revealed that health-risk behaviors (β = 0.445), knowledge of COVID-19 (β = 0.148), gender (β = 0.145), and age (β = 0.133) were predictive of COVID-19 preventive behaviors. Additionally, most respondents (48.9%) had moderate overall effects from COVID-19. Based on our findings, these factors must be considered when implementing public policies to improve COVID-19 preventive behaviors among the currently employed working-age population. In addition, appropriate interventions must be established and evaluated for the pandemic’s long-term effects.