Purpose: The purpose of the study was to estimate the number of new breast cancer cases in France in 2015 attributable to breastfeeding for durations below recommendations (at least 6 months per child), and cases prevented through historical breastfeeding. As a secondary analysis, the corresponding numbers for ovarian cancer were estimated. Methods: Historical breastfeeding data were obtained from population surveys. Duration of breastfeeding data were obtained from the French Épifane cohort study. Relative risks were obtained from meta-analyses, cohort, and case–control studies. Cancer incidence data were obtained from the French Network of Cancer Registries. A 10-year latency period was assumed. Results: Among parous women 25 years of age and older, 14.1% breastfed for at least 6 months per child born before 2006. As a result, 1,712 new breast cancer cases (3.2% of all new breast cancer cases) were attributable to breastfeeding for OpenSPiltSPi 6 months per child, while actual breastfeeding practices prevented 765 breast cancer cases. Furthermore, 411 new ovarian cancer cases (8.6% of all new ovarian cancer cases) may be attributable to breastfeeding for OpenSPiltSPi 6 months per child, with breastfeeding preventing 163 ovarian cancer cases. Conclusions: The historically low breastfeeding prevalence and duration in France led to numerous avoidable cancer cases.