Introduction: Emerging adulthood is associated with unique stressors including adaptations to new environments, which have been linked to mental illness. Additionally, the built environment, including buildings, public spaces, other infrastructures, and associated factors, have been linked with altered stress hormone levels and mental health outcomes. Thus, the present study aimed to explore how environmental features including greenspace, air pollution, active living environment (ALE), and marginalization relate to depression and diurnal cortisol secretion patterns among university students during the COVID-19 pandemic. Methodology: First- and second-year Ontario university students (N = 238; Mage = 19.0) completed an online survey comprising the Beck Depression Inventory (BDI) and questions about the built environment. Further built environment factors were obtained based on participants’ postal codes using the Canadian Urban Environmental Health Research Consortium (CANUE) database. Participants completed mail-in biological sample kits and provided saliva samples for later diurnal cortisol determination. Results: Marginalized regions and areas with high ALE scores were correlated with higher levels of air pollution and lower surrounding greenspace. Measures of greenness were negatively correlated with BDI scores. Air pollution predicted elevated morning cortisol levels above and beyond greenspace. Additional analyses revealed that the relationship between greenspace and depressive symptoms was not mediated by cortisol. Conclusions: These findings suggest that individuals’ living environments are tied to mental health and biological stress markers. Public health initiatives should consider features of the built environment and associated factors as determinants of health that may be particularly important considerations among young adults.

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