Relevant demographic and environmental conditions need to be understood before tailoring policies to improve mental health. Using community health survey data from 25 communities in Seoul, 2013, cross-sectional associations between mental health and community level environments were assessed. Mental health outcomes (self-rated stress levels (SRS) and depressive symptoms (DS)) were analyzed. Community environmental factors included green space, green facilities, and annual PM10 level (AnnPM10); socio-demographic factors included sex, age, education, labor market participation, comorbidity, sleep hours, physical activity, smoking, and drinking. A total of 23,139 people with the following characteristics participated: men (44.2%); age groups 19−39 (36.0%), 40−59 (39.4%), 60−74 (19.2%), and 75+ (5.4%). Women had higher odds ratios (OR) for SRS [OR 1.22, 95% Confidence interval (CI) 1.17–1.27] and DS [OR 1.55, 95% CI 1.42–1.71]. Regular physical activity predicted SRS [OR 0.90, 95% CI 0.84–0.95] and DS [OR 0.98, 95% CI 0.88–1.10]; current smoking and drinking were adversely associated with both SRS and DS. Higher accessibility to green space (Q4) was inversely associated with DS [OR 0.89, 95% CI 0.81−0.97] compared to lower accessibility (Q1). AnnPM10, annual levels for particles of aerodynamic diameter <10 µm (PM10), among communities was associated with poorer SRS [OR 1.02, 95% CI 1.00–1.04] by 10 μg/m3 increases. Therefore, both demographic and environmental factors should be considered to understand mental health conditions among the general population.