Pollution of surface water by toxic metals has received considerable attention due to its risk to human health and ecology. Consequently, the present study focused on the assessment of the heavy metal concentrations in the water from Mairua dam. This research reveals the concentrations of the selected toxic metals (Pb, Cd, Co, Cr and Ni) in water samples. The following trend was observed across the sampling points; Co> Cr> Cd> Ni> Pb. The concentrations of Pb across the sampling points were within the WHO (2011) permissible limits. However the Cd concentrations across the sampling points were within the WHO limits of 0.003 mg/kg with exemption of W1, W2, W3, W4, W5, W8 and W10 respectively. Similarly the concentrations of Co in the analyzed water samples were all above WHO (2011) tolerable limits of 0.01 mg/kg. The elevated level of cobalt in the water may be attributed to leachates from farmlands and domestic waste. The concentrations of Cr across the sampling points were within the WHO (2011) permissible limits of 0.005 mg/kg with exemption of samples W2, W3, W4, W7, W8, W10 and W11 respectively. However, the concentrations of Ni across the sampling points were higher than WHO (2011) permissible limits of 0.01 mg/kg. Nickel is introduced into the environment from both natural and man-made sources. The results obtained for Contamination Factors reveals low contamination (Cf<1) with an exemption of Cd in points W1 and W2. PLI denotes no pollution and Hazard quotient values also showed that the analyzed water has no potential health effects with exemption to Cd. The results also shows that there was no significant difference in the analyzed samples at p<0.005 across the sampling sites. Pollution of the water by Cd may be as a result of anthropogenic activities that are released into the dam. It is recommended to periodically monitor the water.