We estimate the impact of indoor face mask mandates and other non-pharmaceutical interventions (NPI) on COVID-19 case growth in Canada. Mask mandate introduction was staggered over two months in the 34 public health regions in Ontario, Canada. Using this variation, we find that mask mandates are associated with a 25 percent or larger weekly reduction in new COVID-19 cases in July and August, relative to the trend in absence of mandate. Province-level data provide corroborating evidence. We control for factors such as mobility (using Google geo-location data) and past cases. Our analysis of additional survey data shows that mask mandates led to an increase of about 30 percentage points in self-reported mask wearing in public. Counterfactual policy simulations suggest that mandating indoor masks nationwide in early July could have reduced new COVID-19 cases in Canada by 25 to 40 percent in mid-August (700 to 1,100 fewer cases per week).