This study reports the combined effects of fly ashes, hemp fibers, and shives on the strength performance of modified mortars. Fly ashes are first processed using high-intensity mechanical milling. Natural fibers are observed using X-ray microtomography to reveal the microstructural arrangement. Preliminary formulations are attempted with different proportions of hemp fibers and shives up to 4%. X-ray diffraction, infrared, and thermal gravimetric analysis (TGA) methods are used to reveal the main structural changes in modified cement pastes. Workability and strength characteristics of various formulations are evaluated for ranking. The best performing modified mortars are selected based on their strength performance to formulate the mix proportions of milled fly ash and cement mortar. The experimental results demonstrate the beneficial effect of adding hemp fibers with shiv to achieve acceptable workability. In addition, it was found that the optimal hemp content in modified mortar was 3%. This amount minimizes the negative effect of natural additions on mechanical strength. Finally, fly ashes are found to be an efficient leverage to formulate modified mortars exhibiting a strength performance superior to that of the reference mortar. (C) 2019 American Society of Civil Engineers.