Background: Low muscle mass (skeletal muscle index, SMI) and density (skeletal muscle density, SMD) are associated with chemotherapy toxicity and shorter survival in women with breast cancer. This has not been studied specifically in women ≤ 40 years at diagnosis. They have different body compositions and more aggressive cancers than older women. We compared pre-treatment muscle measures in survivors and non-survivors and investigated their association with overall survival (OS) and progression-free survival (PFS). Methods: This case-control study included 112 women aged ≤ 40 years at diagnosis. Women with pre-treatment CT scans from 2009-2018 were identified; non-survivors were matched with survivors by age, year of diagnosis and disease characteristics. Body composition was determined by CT analysis. Measures were compared between the groups using Kruskal-Wallis tests. Kaplan-Meier methods summarized OS and PFS and the associations of muscle characteristics with OS and PFS were examined by univariate Cox proportional hazard models. Results: Median age was 35 years; median follow-up was 8.2 years. 75% had Stage II or III disease and 21% Stage IV disease. 33% were sarcopenic (SMI < 40) and 16% had low SMD (HU <37.8). Non-survivors had more intermuscular fat (IMAT), reduced SMD, and reduced skeletal muscle gauge (SMG). Sarcopenia was not associated with OS; however, sarcopenia was associated with shorter PFS. The median skeletal muscle gauge was 1973; low skeletal muscle gauge (SMG <1973) was associated with both shorter OS and PFS. Median IMAT was 1.6, and high IMAT (≥1.6) was associated with shorter OS and PFS. Conclusions: Low muscle mass (sarcopenia) at breast cancer diagnosis was associated with shorter PFS and low muscle density (low SMG and higher IMAT) was associated with shorter OS and PFS in women ≤ 40 years. These sub-optimal muscle characteristics may indicate an overall reduced state of health and/or decreased ability to tolerate treatment, thus reducing survival. Future research should determine the significance of muscle changes throughout treatment and establish standards for improved muscle health.