Background: In the last century, breast cancer incidence and mortality was higher among higher versus lower educated women in developed countries. Post-millennium, incidence rates have flattened off and mortality declined. We examined breast cancer trends by education level, to see whether recent improvements in incidence and mortality rates have occurred in all education groups. Methods: We linked individual registry data on female Norwegian inhabitants aged 35 years and over during 1971–2009. Using Poisson models, we calculated absolute and relative educational differences in age-standardised breast cancer incidence and mortality over four decades. We estimated educational differences by Slope and Relative Index of Inequality, which correspond to rate difference and rate ratio, comparing the highest to lowest educated women. Results: Pre-millennium, incidence and mortality of breast cancer were significantly higher in higher versus lower educated women. Post-millennium, educational differences in breast cancer incidence and mortality attenuated. During 2000–2009, breast cancer incidence was still 38% higher for higher versus lower educated women (Relative Index of Inequality: 1.38, 95% confidence interval: 1.31–1.44), but mortality no longer varied significantly by education level (Relative Index of Inequality: 1.09, 95% confidence interval: 0.99–1.19). Among women below 50 years, however, the education gradient for mortality reversed, and mortality was 28% lower for the highest versus lowest educated women during 2000–2009 (Relative Index of Inequality: 0.72, 95% confidence interval: 0.51–0.93). Conclusions: Post-millennium improvements in breast cancer incidence and mortality have primarily benefited higher educated women. Breast cancer mortality is now highest among the lowest educated women below 50 years.