Currently the rapid population growth in the world has highly aggravated the need for private houses and large infrastructure installation all over the world that tremendously needs the consumption of cement. But the production of Cement is a very high energy-consumptive, environmental unfriendly and consumer of nonrenewable natural resources. Due to its sever impact on environment, cost and energy intensive approach, many studies are in search of getting locally available alternative materials that can reduce cement cost and along with its pollution effect. Taking in to account his study aimed to investigate the partial replacement of cement with Red teff straw ash (RTSA) in cement mortar at 0%, 5%, 10%, 15%, and 20% by weight of cement. The study also investigated the fresh properties of mortar (setting time and workability), physical test for cement and sand, chemical composition of RTSA, mechanical (compressive strength) at 3, 7 and 28 days, ultrasonic pulse velocity (UPV) at 28 and 56 days, durability (sulfate attack, water absorption and porosity) at 28 and 56days, and microstructural property, Fourier transform infrared (FT-IR), Thermogravimetric thermal analysis (TGA) at 7 days and 28 days were conducted and Scanning electron microscope (SEM) analysis at 28 days was conducted. The results revealed both mechanical, durability and microstructural properties of mortar were enhanced at 10% replacement of red teff straw ash when compared to control (0%) mix .But beyond 10% replacement has shown a slight reduction as compared to the control mix where as the workability of mortar decreases with an increasing percentage of replacement due to the high surface area of RTSA. Consequently, uniform and denser morphology was formed, and both durability and micro structure were enhanced at 10% RTSA replacement because of C-S-H increased by consuming calcium hydroxide and decreasing voids in cement grains. Thus, it is recommended that the utilization of agricultural wastes such as RTSA as supplementary cementitious material in mortar production should be encouraged. The statistical significance of durability properties of mortar based on their experimental test results are analyzed by simple linear model and general linear model (MANOVA) with the help of IBM SPSS and origin proV2018. Keywords- Red teff straw ash, Cement, mortar, Compressive Strength, Microstructure and durability