Objectives The purpose of this study was to examine whether UV exposure alters folate status according to C677T‐MTHFR genotype, and to consider the relevance of this to human health and the evolutionary model of skin pigmentation. Methods Total Ozone Mapping Spectrometer (TOMS) satellite data were used to examine surface UV‐irradiance, as a marker of UV exposure, in a large (n = 649) Australian cross‐sectional study population. PCR/RFLP analysis was used to genotype C677T‐MTHFR. Results Overall, cumulative UV‐irradiance (42 and 120 days pre‐clinic) was significantly negatively related to red cell folate (RCF) levels. When the cohort was stratified by MTHFR‐C677T genotype, the relationship between UV‐irradiance (42 days pre‐clinic) and RCF remained significant only in the cohorts containing carriers of the T allele. Statistically significant z‐score statistics and interaction terms from genotype and UV‐irradiance (p‐interaction) demonstrated that genotype did modify the effect of UV‐irradiance on RCF, with the largest effect of UV being demonstrated in the 677TT‐MTHFR subjects. Conclusions Data provide strong evidence that surface UV‐irradiance reduces long‐term systemic folate levels, and that this is influenced by the C677T‐MTHFR gene variant. We speculate this effect may be due to 677TT‐MTHFR individuals containing more 5,10CH2‐H4PteGlu, and that this folate form may be particularly UV labile. Since UV‐irradiance lowers RCF in an MTHFR genotype‐specific way, there are likely implications for human health and the evolution of skin pigmentation.