The study investigated how governance structures influence the organizational performance of firms listed on the Nairobi Securities Exchange (NSE), focusing on board composition and audit committee composition within the theoretical lenses of Agency, Stakeholder, and Stewardship theories. Using a descriptive research design and primary data collected from 138 respondents drawn from a target population of 210 employees, the study applied multiple regression analysis in SPSS (version 28), with validity confirmed through content and face assessment and reliability verified using Cronbach’s alpha (≥0.70). Findings revealed that governance mechanisms significantly affect firm performance individually and collectively, with audit committee composition (β = 0.315) exerting the strongest impact, followed by board diversity (β = 0.193), highlighting the value of specialized oversight, diversity, and independent monitoring. The results reinforce the importance of accountability, transparency, and ESG principles as articulated in the guiding theories. Policy recommendations call for the Capital Markets Authority (CMA) and NSE to enhance governance guidelines through mandated gender and expertise diversity, stronger board independence, ESG disclosure requirements, and routine governance audits. Managerially, firms are encouraged to strengthen board capacity, implement succession planning, provide continuous training, establish specialized audit committees, and conduct annual board evaluations to ensure strategic alignment. The study further recommends future research on governance effects on non-financial performance, comparative studies within the East African region, and longitudinal designs to assess the long-term impact of governance reforms on firm sustainability and performance.