Objectives: In this study, we aimed to assess the effects of complete decongestive therapy (CDT) on reducing lymphedema and enhancing gross motor strength (GMS), functional ability in the upper arm, quality of life (QoL), and pain relief among women who had undergone breast cancer surgery and chemo/radiotherapy in Montenegro. Methods: This prospective observational/pilot study included 50 women with breast cancer-related arm lymphedema, with an average age of 60.88 ± 12.78 years. The four-week Phase1-CDT program involved manual lymphatic drainage, compression bandaging, skin care, tailored kinesitherapy and patient education. Measurements included arm edema circumference compared to the contralateral arm, pain severity (VAS), arm muscle strength (MMT), functional ability (QDASH), and overall QoL (WHOQOL-BREF). Results: Following CDT, significant reductions in lymphedema circumference were observed in various areas and overall (p = 0.002), along with improvements in overall upper-arm GMS (p = 0.002) and specific upper-extremity movements such as wrist and forearm flexion, supination, and external rotation (p < 0.001). Significant improvements were also observed in pain severity and QDASH scores (p < 0.001), and overall QoL significantly increased (p < 0.001). Muscle strength in the hand, wrist, forearm, and shoulder also improved significantly (p < 0.05). We found a negative correlation between edema size and motor function in different muscle groups of the upper extremities, as well as between the QDASH score, quality of life, and overall upper-arm gross motor strength. Conclusions: It was observed that the four-week Phase 1-CDT program significantly improved lymphedema severity, functional abilities, gross motor strength, quality of life, and pain levels in Montenegrin women with breast cancer who had undergone mastectomy and chemo/radiotherapy. Our findings are limited to the immediate post-intervention period. This study is the first of its kind in Montenegro, suggesting the need for future randomized studies with a larger number of participants are needed.