This paper addresses the effects of seasonal and annual rainfall variability on agricultural production that has had great influence on sugarcane biomass yields in Mumias Sub County in Kakamega County of Kenya. Seasonal and annual rainfall variability has adversely affected agricultural production globally. Mumias Sub County experienced rainfall variation between 1982 and 2012. The objective of this paper was to establish the effects of variations in total seasonal rainfall and total annual rainfall amounts on sugarcane biomass yields. A descriptive longitudinal research design was adopted. Document analysis was used to obtain data on rainfall amounts from Mumias weather station and sugarcane biomass yields from Mumias Sugar Company records. Quantitative data on seasonal and annual rainfall variability and sugarcane yield were analyzed using simple linear regression. The results show 55% (r²=0.55) and 40% (r²=0.4) of variation of sugar cane biomass yields can be explained by long seasonal rainfall (March, April, May) and short seasonal rainfall (October, November, December) respectively. Similarly, 61 % (r²=0.61) of variation of sugarcane biomass yield can be explained by total annual rainfall amounts. The study demonstrated that long seasonal rainfall and total annual rainfall amounts are dominant climatic factors accounting for variation of sugar cane biomass yields in Mumias. Recommendations made to farmers are they align activities of sugarcane production with temporal rainfall variability by observing planting dates, apply fertilizers when heavy rains sub size and apply mitigating measures such as irrigation to supplement water supply to sugarcane crops during dry seasons and grow more of drought resistant varieties.