To improve causality, this thesis used the counterfactual framework to develop two novel and statistically robust approaches to analyse the effect of urban greenspace on mental health. The first approach was a cross-sectional assessment that used statistical matching in addition to regression modelling to establish the effect of local public greenspace on a person’s mental health for those with and without a private garden. The second approach used longitudinal data in a Before-After Control Intervention study design to establish the effect of the change in different greenspace characteristics on mental health when a person moved between urban areas. Both these approaches were applied to the British Household Panel Survey – a nationally representative survey of Great Britain containing individual-level information on mental health and the socio-economic confounders of mental health. Findings from the first approach suggested that the effect of access to private greenspace on mental health outweighs the beneficial effects of access to public greenspace. Specifically, having a private domestic garden substantially reduced the maximum probability of poor mental health for men and women, regardless of their access to local public greenspace. The second approach highlighted the importance of greenspace quality and proximity for mental health. Bird species richness and distance to nearest greenspace, proxy measures for greenspace quality and proximity respectively, provided the most inference when modelling the effect of change in greenspace characteristics on mental health. Comparatively, measures of greenspace quantity and recognised standards and guidelines of greenspace access provided less inference than a model that did not include a measure of greenspace. Given these results, greenspace quality, proximity and access to private gardens should be a priority for future policies to improve the status of both urban greenspace and mental health in Great Britain.