Background: Experiments and large-scale epidemiological studies indicate the importance of green space for mental health. However, little research has been conducted to elucidate whether these mental health benefits are more dependent upon the quantity or quality of the green space. Methods: Symptoms of psychological distress were measured in 3897 women who did not change neighbourhood up to 15 years postpartum using the Kessler 6 psychological distress scale from 2004 onwards. The percentage land-use of the neighbourhood was used to ascertain a measure of green space quantity. A Likert scale was used to measure green space quality in response to the statement "there are good parks, playgrounds and play spaces in this neighbourhood." Multilevel negative binomial growth curve regression models were used to examine the patterning of symptoms of psychological distress across the postpartum period in relation to green space quantity and quality, adjusting for person-level and geographical markers of confounding. The same variables were also fitted in multilevel logistic regressions to examine the odds of reporting serious mental illness (as defined by K6 scores ≥ 13 out of 24). Results: Symptoms of psychological distress were fewer among women who agreed (rate ratio (RR) 0.95, 95%CI 0.91 to 0.98) and strongly agreed (RR 0.89, 95%CI 0.85 to 0.93) local parks were good quality. The odds of reporting serious mental illness were also lower among women who agreed (odds ratio (OR) 0.88, 95%CI 0.77 to 1.00) and strongly agreed (OR 0.74, 95%CI 0.64 to 0.86) local parks were good quality. No association was found between green space quantity and symptoms of psychological distress or the odds of reporting serious mental illness. Conclusions: This study suggests it may be how mothers perceive green space nearby and what those spaces enable them to do, rather than simply how much there is overall, that is important for promoting mental health in the postpartum period. In conclusion, community consultation is likely to be a crucial part of strategies that maximise the health benefits of urban greening for everyone.