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Biennial breast screening enables timely diagnosis, a critical determinant of survival. However, this benefit depends on navigating complex healthcare systems where immigrant women often encounter barriers. We posed the question of whether immigrant women in Ontario experience a “double burden” of lower screening adherence and prolonged intervals between clinical presentation and confirmed diagnosis compared to long-term residents. We identified women aged 50–74 eligible for breast screening in provincial administrative databases. We calculated the proportion up-to-date with biennial mammography annually (2012–2020) and modelled the incidence rate ratio (IRR) using negative binomial regression, adjusting for age, marginalization, and morbidity. Additionally, we identified women with a first breast cancer diagnosis and matched each immigrant 1:2 to long-term residents by age, stage, and year of diagnosis. The time from first clinical presentation to diagnosis was analyzed using adjusted linear regression and conditional logistic regression. On January 1, 2020, 59.9% of long-term residents were up-to-date with screening compared to 51.4% of immigrants (adjusted IRR 0.74; 95% CI 0.71–0.78). The adjusted mean time to diagnosis was 1.21 days longer for immigrants (95% CI 0.10–2.32), a clinically negligible difference. However, diagnostic delays were systemic: 25% of all women waited ≥ 60 days, and 10% waited ≥ 135 days, with no significant difference between groups. Immigrants were more likely to be diagnosed at Stage 2 or 3 compared to long-term residents (51.7% vs. 46.6%, p < 0.001). Comprehensive process mapping of screening and diagnostic processes is required involving representatives of all stakeholders and all subpopulations eligible for breast screening, in order to conduct and implement knowledge-based revisions of screening and BC diagnosis service delivery in Ontario, improve the uptake of screening and the BC stage distribution among immigrant women, and reduce lengthy diagnostic delays.