Nairobi City Water and Sewerage Company has been issuing notices to the public regarding its inability to fully satisfy customer needs. Yet, the company has formulated and implemented strategies to enhance its performance in offering quality service and increasing customer satisfaction. The purpose of this study was to investigate the effect of strategy implementation on performance of Nairobi City Water and Sewerage Company. It was guided by the following specific objectives; to investigate the effect of leadership, culture and organizational structure on strategy implementation and performance, and further establish the moderating effect of external environment on the relationship between strategy implementation and performance of Nairobi City Water and Sewerage Company. The research had negative hypotheses which were derived from the objectives and measured at 95% confidence level. This study was anchored on Mintzberg theory, Resource based theory and McKinsey’s 7S theory as its foundations for theoretical analysis on the connection between the variables. Descriptive and explanatory research designs were used to describe NCWSC’s performance by the former and show the associative relationship between the variables by the later. The target population was 324 and a sample size of 175 respondents was selected through a stratified and simple random sampling techniques. Data was collected a close- ended questionnaire using a 5- point Likert scale. Validity of the questionnaire was determined through carrying out a pre-test of the questionnaire and Cronbach’s alpha was used to test for reliability. Data was analysed using descriptive statistics including frequencies, percentages, mean and standard deviation. Inferential statistics using Pearson’s Product Moment Correlation Coefficient (PPMCC) was used to test the nature and strength of the relationship between strategy implementation and performance using (r) as the regression coefficient. It was also be used to establish how each independent variable affects the dependent variable. The coefficient of determination (r2 ) was used to show the variation in the dependent variable (performance) which is explained by the independent variable (strategy implementation). Multiple regression analysis was used to determine the effect of the relationship between strategy implementation on performance; multiple regression analysis was used to further explain the amount of variation in the dependent variable against independent variables. The study established that strategy implementation has an effect on performance to a great extent. In addition, the researcher established that external environment has an effect on the relationship between strategy implementation and performance in terms of: efficiency, quality service and innovation not including customer satisfaction. The study concluded that the company has done a commendable job in implementing its strategies and recommended that the company should look into improving its customer satisfaction levels to remain competitive by employing more policies.