OBJECTIVE To assess associations of ambient UV with cardiovascular and cancer mortality across US Health Service Areas (HSAs), prioritise outcomes by signal and attributable burden, assess racial and ethnic variation, with UK Biobank corroboration. DESIGN Ecological analysis of HSAs using linear regression of crude mortality on ambient UV and social vulnerability, with correction for multiple comparisons. SETTING US HSAs and UK Biobank. DATA SOURCES Publicly available US data and UK Biobank data accessed under Application 82526. PARTICIPANTS 704 808 annual cardiovascular deaths (aged [≥]65) and 565 287 annual cancer deaths (aged [≥]50) from populations of 52.8 and 115.2 million; UK Biobank analyses included 412 665 individuals. MAIN OUTCOME MEASURES Cardiovascular, all-cancer, and body-site-specific cancer mortality; regression slope per interquartile-range increase in UV ({beta} UV/IQR), modelled attributable burden for US data, and hazard ratios for the UK Biobank data. RESULTS Higher UV was associated with lower cardiovascular ({beta}UV/IQR -68.3 deaths/100 000; P