Ionic liquids (ILs) are innovative and effective solvents for pretreating lignocellulose biomasses because they bring about a noticeable increase in the enzymatic saccharification of these materials. However, the reported solid loadings in ILs of approximately 5.0 wt% reflect their large consumption per gram of biomass and hinder their use in biomass pretreatment. In the present study, a twin-screw extruder with high shearing force was used as a pretreatment reactor to process sugarcane bagasse at high loadings in the IL 1-ethyl-3-methylimidazolium acetate. This procedure allowed effective pretreatment of biomasses at loadings as high as 25 wt% for 8 min at 140 °C, resulting in glucose yields of more than 90% after 24 h of enzymatic saccharification of the pretreated material. This glucose yield was comparable to that of a 4.8 wt% bagasse loading that was pretreated for 120 min at 120 °C in a stirring reactor and enzymatically hydrolyzed under the same conditions. A higher bagasse loading, 50 wt%, also afforded a high glucose yield of 76.4%. Characterization of the pretreated materials by examining the surface morphology at the nanoscopic scale, measuring the specific surface area (SSA), and analyzing the degree of crystallinity showed that the use of the extruder as a pretreatment reactor significantly decreased the crystallinity and increased the SSA by more than 100-fold. In addition, the extrusion process can be conducted continuously and is appropriate for industrial-scale biomass processing, allowing higher reactant concentrations and throughputs, higher mixing rates, and more uniform products compared to batch processes.