Objective: This study examined the prevalence, associated factors, and psychiatric comorbidities of adult ADHD during the COVID-19 pandemic by analyzing nationally representative data. Methods: Among the 5,511 respondents of the 2021 National Mental Health Survey, South Korea, 2,764 (18–49 years) were selected. The Korean versions of the Adult ADHD Self-Report Scale, Composite International Diagnostic Interview, and Structured Clinical Interview for Internet Gaming Disorder were used as diagnostic assessments. Results: The 6-month prevalence of positive screens for adult ADHD was 3.1%. In individuals aged 18 to 29 years exhibiting adult ADHD symptoms, high prevalence rates of alcohol use, depression, and Internet gaming disorders were observed in the last 12 months. Adults who screened positive for ADHD reported significantly lower life satisfaction and resilience, and higher loneliness and social isolation (vs. non-ADHD group). Conclusion: Adult ADHD symptoms significantly influenced mental well-being, highlighting the need for an appropriate treatment/prevention system, particularly for individuals aged 18 to 29 years.