Succession planning is an intended, strategic process through which organisations identify, develop, and retain employees capable of assuming critical leadership positions in future. With the rapid pace of technological changes, globalisation, and increased competition for talent, organisations may face unprecedented risks if leadership continuity is not managed proactively. Though there are many studies on succession planning, studies that are based on a systematic literature review of succession planning as a strategic instrument for organisational sustainability remain limited. This systematic review synthesises evidence from empirical and conceptual studies spanning 1950 to 2025 that investigates the role that succession planning plays in improving employee retention, organisational continuity, and sustainability. The review follows the guidance of Rothwell's Succession Planning and Management (SPM) Model in exploring the conceptual evolution of succession planning, theoretical underpinning, modern practices, and interrelationship of succession planning and its bearing on employee retention and organisational continuity. The review has identified that those organisations which institutionalise succession planning as a strategic capability rather than treating it as merely a reactive or administrative activity achieve better retention rates, preserve institutional knowledge, and increase resilience against uncertainty. It is identified that effective succession planning addresses the risks of leadership discontinuity, while, on the other hand, develops structured development routes and career prospects that enhance employee engagement, commitment, and retention. The paper ends by declaring that succession planning needs to be made an institutionalised strategic organisational capacity and integrated with talent management and human resource strategies to guarantee continuity and growth sustainably.