Background -There is limited evidence of the impact of policies to promote work-family balance on family health. Exploiting the introduction of the UK Flexible Working Act (2003), we examined whether a policy that grants parents the right to request flexible work influences their health and well-being. Methods Using the UK Millennium Cohort Study, we focus on 6424 mothers employed in 2001-2002, when the cohort child was 9 months old, until their child’s seventh birthday. We used a difference-in-differences (DiD) approach to compare changes in outcomes before and after the policy among mothers most likely to benefit and mothers unlikely to benefit from the policy. Results Flexible working increased in a small group of mothers (n=548) whose employer did not offer work flexibility before the reform (treatment group). By contrast, among mothers whose employer already offered flexible work before the reform (control group, n=5810), there was little change or a slight decline in flexible working. DiD estimates suggest that the policy was associated with an increase in flexible working (37.5 percentage points, 95% CI 32.9 to 41.6), but it had no impact on self-rated health (-1.6 percentage points, 95% CI -4.4 to 1.1), long-term illness (-1.87 percentage points, 95% CI -4.3 to 0.5) or life satisfaction scores (β=0.04, 95% CI -0.08 to 0.16). Conclusion – The Flexible Working Act increased flexible working only among a small group of mothers who had not yet the right to request work flexibility, but it had no impact on their health and well-being. Policies promoting work flexibility may require stronger incentives for both parents and employers.