In April 2022, mandatory kilocalorie (kcal) labelling in the out-of-home food sector was introduced as a policy to reduce obesity in England. Here we examined whether the implementation of this policy was associated with a consumer behaviour change. Large out-of-home food sector outlets subject to kcal labelling legislation were visited pre- and post-implementation, and customer exit surveys were conducted with 6,578 customers from 330 outlets. Kcals purchased and consumed, knowledge of purchased kcals and reported noticing and use of kcal labelling were examined. The results suggested that the introduction of the mandatory kcal labelling policy in England was not associated with a significant decrease in self-reported kcals purchased (B = 11.31, P = 0.564, 95% confidence interval (CI) −27.15 to 49.77) or consumed (B = 18.51, P = 0.279, 95% CI −15.01 to 38 52.03). Post-implementation, participants underestimated the energy content of their purchased meal less (B = 61.21, P = 0.002, 95% CI 21.57 to 100.86) and were more likely to report noticing (odds ratio 2.25, P < 0.001, 95% CI 1.84 to 2.73) and using (odds ratio 2.15, P < 0.001, 95% CI 1.62 to 2.85) kcal labelling, which may have wider public health implications. © The Author(s) 2024.