Background Mammographic screening has been used to reduce breast cancer mortality worldwide and remains the main modality for the early detection of this disease. Women from low- and middle-income countries still lack access to periodic mammograms and efficient health care. This cross-sectional study aimed to explore opportunistic mammographic coverage in Brazil, while considering the privately insured population and its association with early breast cancer (EBC) detection. Methods Data on population, gross domestic product (GDP), number of mammograms performed under the Sistema Único de Saúde (SUS) public health system or private system, and women diagnosed with early-stage breast cancer from 2010 to 2019 were retrieved from publicly available databases. Results A total of 39 555 636 mammograms with an average of 3 955 564 ± 395 704 mammograms were obtained per year from 2010 to 2019 in Brazil. Most examinations (58.6%) were performed in the target population (50-69 years old), while 32% were performed in women aged 40-49, and 9.4% were performed in women <40 years or >70 years of age. The 10-year mammogram coverage was 30.6% in the target population and 24.8% in the population aged 40-49 years, with significant variation across states and municipalities. The overall EBC detection rates in Brazil were 30.6% in populations aged 50-70 and 24.8% in those aged 40-50 years. We observed a positive correlation between coverage and EBC detection rate (r = 0.68; P = 0.0001 (50-70 years) and r = 0.75; P < 0.0001 (40-50 years)). According to the GDP, the municipalities with higher GDP per capita had higher mammogram coverage ( P < 0.0001). Conclusions The coverage of mammographic screening for women under the SUS is far below the international guidelines. Additionally, a significant number of mammograms have been performed in non-target populations. This scenario reflects the problematic screening programs in developing countries and reflects low rates of EBC diagnosis. As Brazil is a continental country with heterogeneous socioeconomic indicators, we observed significant variations in the number of mammograms performed by age groups when separated by states and municipalities. Even when considering supplemental health system coverage, municipalities with higher GDP per capita were associated with higher mammogram coverage.