Purpose <p>Obesity and low muscle mass (LMM) have been associated with adverse outcomes in cancer population. However, their individual and combined effects on long-term mortality and cardiovascular disease (CVD) outcomes in breast cancer patients are not well characterized.</p> Methods <p>We conducted a retrospective study of 46,037 women aged 40&#xa0;years and older with pathologically confirmed breast adenocarcinoma using data from the Cancer Public Library Database. LMM was defined as the lowest quartile of appendicular skeletal muscle mass index (ASMI), estimated by a validated equation. The remaining quartiles were classified as normal muscle mass (NMM). Obesity was defined as BMI ≥ 25&#xa0;kg/m². Patients were categorized into four groups by muscle mass and obesity status. Multivariable Cox proportional hazards models assessed associations with overall, cancer-specific, and cardiovascular mortality, and incident CVD.</p> Results <p>Over a mean follow-up of 4.63&#xa0;years, 2,286 deaths and 851 incident CVD events were recorded. Compared to NMM, LMM was independently associated with increased overall (adjusted hazard ratio [aHR] 1.27, 95% CI 1.15–1.40), cancer-specific (aHR 1.19, 95% CI 1.08–1.32), and cardiovascular (aHR 1.76, 95% CI 1.10–2.81) mortality. When stratified by muscle-obesity status, the LMM with obesity group had the highest risk of overall (aHR 1.69, 95% CI 1.18–2.42) and cancer (aHR 1.58, 95% CI 1.03–2.43) mortality. LMM without obesity was also associated with increased overall, cancer, and cardiovascular mortality.</p> Conclusions <p>LMM was independently associated with increased mortality in breast cancer patients, with risk amplified in those with coexisting obesity.</p>