BackgroundUrban greenspaces might reduce non-communicable disease risk. The links between greenspaces and mortality related to non-communicable disease remain unclear. We estimated the associations between residential greenspace quantity and access with all-cause, cardiovascular disease, cancer, respiratory, and type 2 diabetes mortality. MethodsWe linked 2011 UK Census data of London-dwelling adults (aged ≥18 years) to data from the UK death registry and the Greenspace Information for Greater London resource (N=4 645 581). We calculated percent greenspace area, access point density (access points per km2), and distance (metres) to the nearest access point for each respondent's residential neighbourhood (defined as 1000-m street network buffers) for greenspaces overall and by park type using a geographic information system. We estimated associations using Cox proportional hazards models, adjusted for a range of confounders. Ethics approval was granted by the London School of Hygiene and Tropical Medicine's Observational/Interventions Research Ethics Committee. FindingsRespondents were followed for 8·4 years (SD 1·4). A one percentage point increase in pocket park area was associated with a 6% decrease in all-cause mortality risk (hazard ratio [HR] 0·94 [95% CI 0·92–0·97]) and an increase of ten pocket park access points per km2 was associated with an 8% decreased respiratory mortality risk (0·92 [0·85–0·99]). Other associations were observed but the estimated effects were 3% or less. For example, all-cause mortality risk for increases of one percentage point in regional park area (0·99 [0·99–1·00]) and increases of ten small open space access points per km2 (1·03 [1·02–1·03]). InterpretationIncreasing pocket park quantity and access might mitigate premature mortality risk. More research is needed to elucidate the mechanisms that could explain these associations. FundingHealth Data Research UK.