Purpose <p>The purpose of the study was to elucidate life-course and current exercise levels among women aged 50–69 who participated in BreastScreen Norway during the period from 2006 to 2015, and to estimate the risk of breast cancer based on exercise trajectory classes and current exercise level.</p> Methods <p>Self-reported data from 325,953 women were utilized for descriptive and exploratory analyses. Retrospective information about exercise levels at ages 15–19, 20–29, 30–39, and 40–49 was used to establish exercise trajectory classes, while reported exercise at ages 50–69 was considered as current activity. Hazard Ratios (HR) with 95% confidence intervals (CIs) were estimated using Cox proportional hazards models adjusting for age, educational level and body mass index (BMI).</p> Results <p>Adjusted HR of breast cancer was 0.92 (95% CI: 0.87–0.98) for women in the stable high class and 0.94 (95% CI: 0.89–0.99) for those in the increasing trajectory class compared to the stable low class. The HRs was 0.89 (95% CI: 0.82–0.97) for 4.5&#xa0;h current exercise per week and 0.72 (95% CI: 0.62–0.84) for ≥ 6&#xa0;h, compared to those reporting no current exercise. Statistically significant results were observed for asymptomatic (screen-detected) and symptomatic cancers. Stratification by BMI and smoking status did not reveal substantial differences in the risk estimates.</p> Conclusion <p>Engaging in high-level exercise throughout one's lifetime significantly reduces the risk of breast cancer. This study highlights the importance of maintaining or increasing physical activity levels throughout life to mitigate breast cancer risk.</p>