Vanderbilt University School of Medicine looked at the current COVID outbreak in Tennessee and broke the hospitalization numbers down by the counties patients were coming from and whether those counties had masking mandates. The results are stark. The growth in hospitalizations is greatest in counties without masking requirements. Indeed, the inverse relationship between masking and hospitalization lines up across the spectrum from areas with little masking to those where mandates are widespread. You can see the discussion of the study here.
But this chart tells the story pretty clearly.