I investigate the prevalence of family instability in the United Kingdom and its association with children's school readiness at age 5. Data are from three sweeps of the Millennium Cohort Study (2001-2007). Family instability is measured by mother's self-report of union status changes since her child's birth. Outcome measures include mother assessments of child behavior and standardized scores on cognitive assessments. Maternal education and household income explained the association of family instability with children's emotional behavior and nonverbal ability, but conduct problems and verbal ability remained associated with family instability after accounting for explanatory factors. Compared to children born to married parents, the verbal ability of children born to cohabiting parents was less affected by later family structure change.