Associations between parks and mental health have typically been investigated in relation to the presence or absence of mental illness. This study uses a validated measure of positive mental health and data from RESIDential Environments (RESIDE) Project to investigate the association between the presence, amount and attributes of public green space in new greenfield neighbourhood developments and the mental health of local residents (n = 492). Both the overall number and total area of public green spaces were significantly associated with greater mental wellbeing, and findings support a dose-response relationship. Positive mental health was not only associated with parks with a nature focus, but also with green spaces characterised by recreational and sporting activity. The study demonstrates that adequate provision of public green space in local neighbourhoods and within walking distance is important for positive mental health.