Theories of adult development and aging suggest that older adults turn towards religion and spirituality (R/S) to increase meaning and purpose in life, and to assist in coping with adverse experience. We sought to examine the relationship between R/S and positive mental health and mental illness (i.e., psychological distress) in adulthood, and to determine the potential moderating role of age. Data from the 2012 Canadian Community Health Survey - Mental Health (CCHS-MH), a national population health study, were used. A multivariable linear regression model revealed a significant R/S by age interaction for positive mental health (p = 0.001). Although R/S was associated with positive mental health among all participants, there was a stronger relationship between R/S and positive mental health for older adults. No statistically significant relationship between R/S and psychological distress was observed. Findings highlight the importance of R/S to positive mental health across the adult lifespan. The differential relationships among R/S, positive mental health, and mental illness underscore the utility of using the dual-continua of mental health and mental illness in order to understand their respective determinants.