Objective Mental health is a global concern and needs to be studied more closely. We aimed to estimate the prevalence of mental disorders and their associated factors among the general population in Korea. Methods The National Mental Health Survey of Korea 2021 was conducted between June 19 and August 31, 2021 and included 13,530 households; 5,511 participants completed the interview (response rate: 40.7%). The lifetime and 12-month diagnosis rates of mental disorders were made using the Korean version of the Composite International Diagnostic Interview 2.1. Factors associated with alcohol use disorder (AUD), nicotine use disorder, depressive disorder, and anxiety disorder were analyzed, and mental health service utilization rates were estimated. Results The lifetime prevalence of mental disorders was 27.8%. The 12-month prevalence rates of alcohol use, nicotine use, depressive, and anxiety disorders were 2.6%, 2.7%, 1.7%, and 3.1%, respectively. The risk factors associated with 12-month diagnosis rates were as follows: AUD: sex and age; nicotine use disorder: sex; depressive disorder: marital status and job status; anxiety disorder: sex, marital status, and job status. The 12-month treatment and service utilization rates for 12-month AUD, nicotine use disorder, depressive disorder, and anxiety disorder were 2.6%, 1.1%, 28.2%, and 9.1%, respectively. Conclusion Approximately 25% of adults in the general population were diagnosed with mental disorders during their lifetime. The treatment rates were substantially low. Future studies on this topic and efforts to increase the mental health treatment rate at a national level are needed.