This comprehensive review examines the evolving and multidimensional roles of nursing in the management of complex and chronic health conditions. As global healthcare systems face increasing burdens from long-term illnesses, multimorbidity, and aging populations, nurses play a pivotal role in ensuring continuity, coordination, and quality of care. The review synthesizes current evidence on nursing-led and nursing-integrated care models, including case management, chronic disease self-management support, transitional care, community-based nursing, and advanced practice nursing. Key outcomes explored include patient clinical indicators, quality of life, treatment adherence, healthcare utilization, and system-level efficiency. The review further highlights mechanisms through which nursing interventions influence outcomes, such as patient education, care coordination, psychosocial support, and interprofessional collaboration. Emerging trends—including digital health integration, nurse-led clinics, and expanded scopes of practice—are also discussed. Findings underscore that effective nursing engagement significantly improves outcomes for individuals with complex and chronic conditions while enhancing healthcare system sustainability. The review concludes with implications for practice, policy, and future research, emphasizing the strategic importance of nursing in chronic care transformation.