Background: Lymphedema of the arm is one of the most common complications following breast cancer surgery. Aims: The primary aim of this study was to evaluate the effects of complex decongestive physical therapy (CDPt) on upper extremity functions, activities of daily living (ADL), and quality of life (QoL), and secondly the effects of the degree of lymphedema on post-treatment differences in older patients with breast cancer-related lymphedema (BCRL). Subjects and Methods: Sixty-eight patients who had BCRL were included between 2015 and 2017. Arm function was evaluated with the Constant-Murley scale, while ADL was measured with the Lawton Instrumental Activities of Daily Living Scale, and QoL was measured with the Lymphedema Functioning, Disability and Health Questionnaire as pre- and post-treatment tests. The patients underwent a CDPt program for 6 weeks. Results: There were statistically significant improvements for all outcome measurements in older patients with Grade 1 and 2 lymphedemas after the treatment (P < 0.001). The Grade 1 patients had a greater difference at mobility, participation in the life and social activities, and their total scores of quality of life had a significance level of P < 0.001. Conclusion: Older patients with Grade 1 BCRL had better mobility, participation in the life, and social activities. CDPt provides enhancement of arm functions, ADL, and QoL in older patients with breast cancer-related lymphedema.