More recent guidelines are more supportive for post‐mastectomy radiation in all node‐positive breast cancer patients. We examined the rate and predictors of post‐mastectomy radiation receipt in Ontario Canada from 2010 to 2014. Of 6535 node‐positive post‐mastectomy patients, 73.9% received radiation. The rate was 68.7% (2903/4227) among women with 1‐3 positive nodes. Radiation was less likely to be administered to women who were older, had high levels of comorbidity, or presented with early stages of breast cancer. Regional practice variation was reassuringly modest.