Background. The time between booking an appointment and the appointment taking place, known as lead time, has been identified as a predictor of cancellation and no-show probability in health care settings. Understanding the impact of reducing permissible lead times, that is, the booking horizon, at a policy level in an outpatient care setting is important when mitigating costly cancellation and no-show rates. Few studies have researched this in an observational or experimental setting. Methods. We leveraged longitudinal observational data from an outpatient pediatric rehabilitation organization in Ontario, Canada, consisting of 73,482 visits between June 2021 and October 2023. This organization reduced its booking horizon at the policy level from 12 to 4 wk in February 2023. Using 2 interrupted time-series approaches, we estimated the change in level, slope, and variance of the weekly combined last-minute cancellation and no-show rate associated with the policy change. Results. It is estimated that reducing the booking horizon is associated with an absolute reduction in the weekly rate of last-minute cancellations and no-shows of 1.02% to 1.85% (a relative reduction of 8.07%–15.70%). Furthermore, the variance dropped by 48.18%. Conclusion. Reducing the appointment booking horizon is associated with a significant reduction in the rate and variance of costly last-minute cancellations and no-shows. The reduced variance can also help enable effective usage of strategies such as overbooking for organizations seeking further approaches to mitigating the negative effects of no-shows. Highlights This study uses interrupted time-series approaches to assess the effects of reducing the appointment booking horizon at a policy level on last-minute cancellations and no-shows in a pediatric outpatient care setting. Reducing the permissible booking horizon from up to 3 mo to up to 4 wk is associated with a significant reduction in the rate of last-minute cancellations and no-shows. The shortened booking horizon policy is associated with a significant drop in the variance of last-minute cancellations and no-show rates, which is valuable in settings where overbooking occurs.