The mandatory wearing of face masks remains a contentious policy issue during the COVID-19 pandemic. This column evaluates the impact of mask mandates on the spread of COVID-19 in Canada, using the different timings that masks were mandated across the 34 health districts of the province of Ontario. Mask mandates are associated with a 25% or larger weekly reduction in new COVID-19 cases in July and August, relative to the absence of mandates. Requiring indoor masks nationwide in early July could have reduced new COVID-19 cases in Canada by 25%–40% in mid-August, which translates into between 700 and 1,100 fewer cases per week.