Requirements of proof of COVID-19 vaccination were mandated for nonessential businesses and venues by Canada’s ten provinces throughout the fall of 2021. Leveraging variations in the timing of these measures across the provinces, we applied event study regression to estimate the impact the announcement of these measures had nationally on age-specific first-dose uptake in the subsequent seven-week period. Proof-of-vaccination mandate announcements were associated with a rapid, significant increase in first-dose uptake, particularly in people younger than age fifty. However, these behavioral changes were short- lived, with uptake returning to preannouncement levels—or lower—in all age groups within six weeks, despite mandates remaining in place for at least four months; this decline occurred earlier and was more apparent among adolescents ages 12–17. We estimated that nationally, 290,168 additional people received their first dose in the seven weeks after provinces announced proof-of-vaccination policies, for a 17.5 percent increase over the number of vaccinations estimated in the absence of these policies. This study provides novel age-specific evidence showing that proof-of-vaccination mandates led to an immediate, significant increase in national first-dose uptake and were particularly effective for increasing vaccination uptake in younger to middle-aged adults. Proof-of- vaccination mandates may be effective short-term policy measures for increasing population vaccination uptake, but their impact may differ across age groups.