Objective: Shoulder pain is a common post-mastectomy complication. This research aims to evaluate the role of shoulder structure changes in developing shoulder pain in post-mastectomy patients and determine ultrasonographic findings. Methods: Medical records of post-mastectomy patients with ipsilateral shoulder pain were reviewed. Ultrasonographic findings, visual analogue scale (VAS), Shoulder Pain and Disability Index (SPADI), age, body mass index, time from mastectomy, operation type, lymph node dissection type, presence of lymphedema, and radiation therapy were checked. Results: Of 40 patients, rotator cuff tear, subacromial-subdeltoid bursitis, calcific tendinitis, biceps brachii tendon sheath fluid accumulation, and tendinopathy were seen in ultrasonography. Age was statistically higher in the rotator cuff tear group than non-rotator cuff tear group. Time from mastectomy, VAS, and SPADI were statistically correlated with biceps brachii sheath fluid accumulation. Shoulders with tendinopathy developed pain shortly after surgery. VAS and SPADI were statistically correlated with presence of subscapularis or supraspinatus tendinopathy. Nine of thirteen patients (69.2%) with subscapularis tendinopathy had post-mastectomy lymphedema. Only two of fifteen (18.2%) patients with supraspinatus tendinopathy had a history of radiation therapy. Conclusion Shoulder pain and rotator cuff tendinopathy may occur soon after surgery; therefore, shoulder range of motion exercises and muscle strengthening exercises are necessary in the early days after surgery. Elderly patients have a risk of rotator cuff tear, which physicians should pay more attention to. Even though there is no history of lymphedema and radiation therapy, shoulder lesions may occur. Therefore, prevention of shoulder pain in all patients after breast cancer surgery is needed.