This paper examines the impact of ownership structure and concentration on firm performance in Sri Lanka, an emerging market in Asia. The study estimates a series of regressions using pooled data for a sample of Sri Lankan-listed firms to investigate the impact of ownership concentration and structure on firm performance based on agency theory framework, using both accounting and market-based performance indicators. The results of the study provide evidence for a strong positive relationship between ownership concentration and accounting performance measures. This suggests that a greater concentration of ownership leads to better performance. However, we found no significant impact using market-based performance measures, which suggests the existence of numerous market inefficiencies and anomalies. Furthermore, the findings of the study show that ownership structure does not have a significant distinguishable effect on performance.