This study aimed to evaluate the effects of an early shoulder joint functional exercise program on shoulder joint function and quality of life among breast cancer survivors who underwent modified radical mastectomy. Breast cancer remains the most prevalent malignancy among women worldwide. With advances in early detection and comprehensive treatment, survival rates among breast cancer survivors have markedly improved. Consequently, the focus of postoperative care has shifted from disease control to long-term functional recovery and quality of life enhancement. However, shoulder joint dysfunction following modified radical mastectomy (MRM) remains a common and debilitating complication, significantly affecting upper limb function, daily activities, and overall quality of life. Although early shoulder joint functional exercises have been reported to be beneficial, their systematic integration into routine postoperative nursing care and their effects on both shoulder function and quality of life require further empirical investigation. A quantitative, quasi-experimental, non-equivalent control group design was employed. Sixty female breast cancer survivors who underwent unilateral MRM were recruited from a tertiary hospital in Shandong Province, China, and were allocated into an experimental group (n = 30) and a control group (n = 30) using sequential recruitment and sealed draw assignment. The experimental group received a structured, phase-based early shoulder joint functional exercise program beginning 1–3 days postoperatively, while the control group received routine postoperative functional exercise guidance. Shoulder joint function was assessed using the Constant–Murley Score (CMS) at baseline (preoperative), postoperative day 10, and postoperative day 30. Quality of life was measured using the Functional Assessment of Cancer Therapy–Breast (FACT-B) at postoperative day 30. Data were analyzed using descriptive statistics, independent and paired t-tests, and correlation analysis. Baseline comparisons revealed no statistically significant differences in demographic characteristics or shoulder function scores between the experimental and control groups of breast cancer survivors (p > 0.05), indicating that the two groups were well-matched at the outset of the study. Following the intervention, shoulder function improvement in the experimental group was significantly greater than that in the control group. For instance, assessments using the Constant–Murley Shoulder Score (CMS) demonstrated that the experimental group exhibited statistically significant increases in shoulder function scores at both postoperative day 10 and day 30 compared to baseline levels (p < 0.05), with a consistent upward trend. In contrast, although the control group also showed improvement at the same time points, the extent of improvement was markedly smaller. Intergroup comparisons further confirmed that CMS scores in the experimental group were significantly higher than those in the control group at both postoperative day 10 and day 30 (p < 0.05). With respect to quality of life, as evaluated by the Functional Assessment of Cancer Therapy–Breast (FACT-B) scale, the experimental group exhibited significantly higher total quality-of-life scores than the control group at postoperative day 30 (p < 0.05). Improvements were particularly notable in the physical well-being and functional well-being domains. Further correlation analysis indicated a significant positive relationship between shoulder function scores and overall quality-of-life scores among breast cancer survivors who received early shoulder rehabilitation interventions (p < 0.05). This suggests that greater improvement in shoulder function is associated with better self-perceived quality of life in this population. Early shoulder joint functional exercise significantly improved shoulder joint function and quality of life among breast cancer survivors after modified radical mastectomy. The findings support the systematic incorporation of structured early shoulder joint functional exercises into routine postoperative nursing care to promote functional recovery and enhance overall quality of life.