In developing countries, International Non-Governmental Organizations (INGOs) have taken active and complementary roles in sustainable human development. Despite the role of INGOs in development, the issues of sustainability, availability of funds, and supportive leadership have remained a major challenge in developing countries. The concept of competitive advantage has been recognized as a central building block in strategic management and an important precedent to organizational performance. Competitive advantage is a deliberate effort by the organization to align its internal conditions and capabilities with changes in the external environment. This study sought to establish the mediating effect of competitive advantage on the relationship between strategic leadership and performance of international non-governmental organizations in Kenya The study was anchored on strategic leadership theory, upper echelons theory and resource-based view theories. For methodology, descriptive cross-sectional survey design covering 277 respondents was adopted. Semi structured questionnaires were used to collect data. The study findings indicated that data collection tool/questionnaire was reliable as Cronbach alpha was greater than 0.7 for all variables. The results indicated a significant mediating effect of competitive advantage on the relationship between strategic leadership and performance of international non-governmental organizations in Kenya. This means that the influence of strategic leadership on performance is indirect through competitive advantage. The study concluded that both competitive advantage and strategic leadership concurrently influence organizational performance positively. It is recommended that international non-governmental organization must be able to identify their competitive advantage. Keywords: Strategic Leadership, Competitive Advantage, Performance, International Non-Governmental Organizations & Kenya.