The establishment of the UK National Health Service (NHS) in July 1948 was one of the most important health policy interventions of the 20th century, providing universal and free access to medical care and expanding maternal and infant health services. In this paper, we estimate the causal effect of the NHS introduction on early-life mortality and examine whether survival is selective. Using an RD design, we compare individuals born just before and just after July 1948. Leveraging newly digitized weekly death records, we document a significant decline in stillbirths and IMR following the NHS introduction, driven mainly by fewer deaths from congenital conditions and diarrhea. We then use polygenic indexes (PGIs), fixed at conception, to track changes in population composition. Cohorts born at or after the NHS introduction exhibit higher PGIs associated with contextually-adverse traits and lower PGIs associated with contextually-valued traits. Results based on UKBiobank data are robust to family-based designs and replicate in other UK surveys. Effects are strongest in disadvantaged areas and among males, highlighting how large-scale public policies can leave persistent imprints on population composition through selective survival.