This study assessed the influence of Monitoring and Evaluation (M&E) work plan on sustainable water supply in Nyamira South Sub-County, Kenya. The study was based on a theoretical framework composed of monitoring and evaluation work plan of water supply as the independent variable and sustainable water supply as the dependent variable. Frequencies, percentages, mean, and standard deviation; simple and multiple linear regression and Pearson Correlation Coefficient models were applied on the quantitative data while narrative analysis was applied on the qualitative data. The total respondents who agreed and strongly agreed that the water supply in Nyamira South Sub-County was: long enough were 22.7 per cent; adequate for the needs of the people were 22.5 per cent; of good quality were 31.2 per cent; safe for drinking were 32.1 per cent. Those who agreed that funds generated from the water supply were adequate were 15.4 per cent; and that the water was affordable were 22.1 per cent. The qualitative findings suggested that the water supply was not sustainable. Based on the composite mean and standard deviation of 2.47 and 1.01 respectively, the study suggested that the M&E work plan for Nyamira South Sub-County water supply existed and was available. However, the work plan was not in use, was not regularly reviewed nor updated. The study’s hypothesis of no significant relationship between M&E work plan and sustainable water supply in Nyamira South Sub-County, Kenya was rejected since P=0.000<0.05. Strengthening the use of M&E work plan in project implementation was recommended.