Objectives: To investigate overall and breast cancer-specific mortality in early-stage breast cancer patients with and without schizophrenia or related disorders. Methods: We used Danish national registers to identify all women with no prior history of cancer or organic mental disorders, who were diagnosed with early-stage breast cancer 1995–2011. Logistic regression models were used to calculate the odds ratios (ORs) for not being allocated to guideline treatment. Cox regression models were used to compute hazard ratios (HRs) for overall and breast cancer-specific deaths among women allocated or not allocated to guideline treatment. Results: We identified 56,152 women with early-stage breast cancer diagnosed in 1995–2011, of whom 499 women also had been diagnosed with schizophrenia or related disorders. The likelihood of women with schizophrenia or related disorders for not being allocated to guideline treatment was increased (adjusted OR, 1.50; 95% confidence interval (CI), 1.15–1.94). The adjusted HR for all-cause mortality was 1.55; 95% CI, 1.32–1.82 and 1.12 (95% CI, 0.98–1.50) for breast cancer-specific mortality; women allocated to guideline treatment had an adjusted HR for breast cancer-specific death of 1.42 (95% CI, 1.11–1.82). The adjusted HR for death due to unnatural causes was 3.67 (95% CI, 1.80–7.35). Conclusion: The survival of women with schizophrenia or related disorders after breast cancer is significantly worse than that of women without these disorders. These patients are less likely to be allocated to guideline treatment, and, among those who are, mortality from both breast cancer and other causes is increased.