Agricultural residues and energy crops often contain high contents of alkali metals, chlorine, silica, and other elements that promote slagging, fouling, corrosion, and gas emission during thermochemical conversion (e.g., combustion and gasification). Water leaching is a common method, but not always effective to reduce such elements. Bioleaching by adding microbes to water leaching was introduced to leach cellulosic biomass in this study. Three microbial species including two fungi (Fusarium oxysporum and Aspergillus niger) and one bacterium (Burkholderia fungorum) were selected to leach four lignocellulosic feedstocks such as corn stover, wheat straw, switchgrass, and sorghum. Among three microbes, A. niger was found the most efficient to remove most elements by 80% in 48 h, and sorghum was relatively more amenable to bioleaching. With A. niger, the bioleaching with water to feedstock (w/w) ratio of 25 for 6 h was sufficient to leach K (85%), Cl (90%), Mg (60%), and P (70%) from sorghum. Overall, bioleaching is more efficient than water leaching except for Na. Studies on bioleaching mechanism indicated that the acidification resulted from organic acids produced by A. niger during bioleaching might contribute to the higher leaching efficiency over other microbial species and water leaching.