Objectives: To develop a validated model for evaluating the real-world effectiveness of long-term clinical management strategies for women with germline BRCA1 or BRCA2 pathogenic variants. Methods: A microsimulation model was developed that included a BRCA-specific natural history for breast and ovarian cancer, a clinical framework for carrier follow-up, and cancer risk management strategies (breast screening, risk-reducing mastectomy, and bilateral salpingo-oophorectomy). Adherence rates and outcomes for breast screening and risk-reducing surgery were obtained from BRCA carriers seen through a familial cancer service in Melbourne, Australia. The model was assessed for internal and external validity. The model was used to compare women perfectly adhering to screening recommendations versus actual adherence of the clinical cohort. Results: The model accurately predicted cancer incidence, pathology, and mortality. Using actual adherence for breast screening resulted in additional breast cancer deaths (per 1000 women: BRCA1, 2.7; BRCA2, 1.6) compared with perfect screening adherence. This decreased average life expectancy by 0.30 life-years for BRCA1 and 0.07 life-years for BRCA2. When carriers had access to risk-reducing mastectomy, the benefit from improved screening adherence was not significant. Conclusions: The developed model is a good descriptor of BRCA carriers’ lifetime trajectory and its modification by use of risk management strategies alone or in combination. Evaluations of breast screening in BRCA carriers may overestimate the benefits of screening programs unless adherence is considered. By incorporating real-world clinical practice and patient behavior, this model can assist in developing clinical services and improving clinical outcomes for carriers.