This study aimed to examine the effect of increases in the duration of education on sexual activity in later life using the instrumental variable approach. Cross-sectional data were obtained from the English Longitudinal Study of Ageing, Wave 6 (2012/2013) for the analysis. The effect of the duration of education on a binary variable for sexual activity in the past month was examined by two-stage least squares estimation using the 1947 schooling reform as the instrument. A total of 1493 participants were included. The 1947 schooling reform significantly extended the duration of education by a mean of 0.86 years (95% confidence interval (CI), 0.52–1.20; F = 24.70) for men and 0.81 years for women (95% CI, 0.54–1.09; F = 33.82). The second-stage regression showed that an additional year of education increased the probability of having sexual activity in the past month by 0.16 points (95% CI, 0.07–0.25) for men and decreased the probability of having sexual activity in the past month by 0.18 points (95% CI, − 0.30– − 0.05) for women. The causal mediation analysis revealed that long-standing illness mediated the mechanism between education and sexual activity, but the mediational effect was clearly observed only for men: the indirect effect for men was 0.10 (95% CI, 0.01–0.20; the proportion mediated, 78.1%) and for women was − 0.16 (95% CI, − 1.23–0.90; the proportion mediated, 97.8%), respectively. This study confirmed the causal effect of education on sexual activity. However, the effect for men and women was completely opposite.