Despite evidence of associations between urban green space exposure and mental health, explanatory mechanisms and the role of green space qualities remain unclear. This prospective cohort study (n = 929) examined the distinct relationships of residential public open space (POS) availability and 'greenness' with four-year trajectories of psychological distress in Montreal, Canada. Stress-buffering and main effect mechanisms were tested under the respective hypotheses that POS exposures 1) attenuate the impact of stressful events on psychological distress and 2) protect against psychological distress independently of exposure to stressful events. Results from growth mixture models indicate that residing among 'greener' POS protects against rising distress through both mechanisms. Conversely, POS availability was not associated with trajectories of distress when holding greenness constant. Findings reinforce the need to consider the quality as well as quantity of public open space in urban environments.