Purpose Breast cancer incidence among younger women (under age 50) has increased over the past 25 years, yet little is known about the etiology among this age group. The objective of this study was to investigate relationships between modifiable and non-modifiable risk factors and early-onset breast cancer among three prospective Canadian cohorts. Methods A matched case–control study was conducted using data from Alberta’s Tomorrow Project, BC Generations Project, and the Ontario Health Study. Participants diagnosed with breast cancer before age 50 were identified through provincial registries and matched to three control participants of similar age and follow-up. Conditional logistic regression was used to examine the association between factors and risk of early-onset breast cancer. Results In total, 609 cases and 1,827 controls were included. A body mass index ≥ 30 kg/m 2 was associated with a lower risk of early-onset breast cancer (OR 0.65; 95% CI 0.47–0.90), while a waist circumference ≥ 88 cm was associated with an increased risk (OR 1.58; 95% CI 1.18–2.11). A reduced risk was found for women with ≥ 2 pregnancies (OR 0.76; 95% CI 0.59–0.99) and a first-degree family history of breast cancer was associated with an increased risk (OR 1.95; 95% CI 1.47–2.57). Conclusions In this study, measures of adiposity, pregnancy history, and familial history of breast cancer are important risk factors for early-onset breast cancer. Evidence was insufficient to conclude if smoking, alcohol intake, fruit and vegetable consumption, and physical activity are meaningful risk factors. The results of this study could inform targeted primary and secondary prevention for early-onset breast cancer.