Background: The pectoralis major (PM) and pectoralis minor (PMi) are muscles located in the anterior chest wall. The pectoralis major is a fan-shaped muscle composed of the clavicular and sternocostal heads. Typically, the clavicular head originates from the anterior surface of the medial half of the clavicle. The sternocostal head, located just inferior to the clavicular head, originates from the anterior surface of the sternum, superior six costal cartilages, and aponeurosis of the external oblique muscle. The pectoralis minor lies deep to the pectoralis major, positioned on top of the rib cage. Innervation of these muscles is provided by the medial and lateral pectoral nerves. Case Presentation: During a routine educational cadaveric dissection at the University of Toledo College of Medicine, asymmetrical pectoralis major muscles were identified in an 83-year-old male. Further examination revealed that the sternocostal head of the left pectoralis major was not fully developed. Additionally, the left pectoralis minor muscle was missing, and the left medial pectoral nerve was absent. Discussion: Anomalous development of the pectoralis major is often associated with other musculoskeletal developmental defects, and the clinical presentation can vary depending on the involvement of structures. The PMi acts as a surgical landmark and can also be used as a myo-cutaneous flap in reconstructive surgeries. Understanding this anomaly can provide insights for managing cases of anomalous PM and PMi and have effects for future patients with pectoralis variations.