In the present study, two kinds of deep eutectic solvents (DESs) were facilely prepared and adopted to drastically disturb the recalcitrance of corncobs for further boosting sugar yields and obtaining valorized lignin by-products while reducing treatment times. The DES systems included benzyltrimethylammonium chloride (BTMAC)/lactic acid (LA) and benzyltriethylammonium chloride (BTEAC)/lactic acid (LA), and the reactions were conducted in a short time of 2 h at 100–140 °C. The proposed process desirably retained most of the cellulose in pretreated corncobs (94.1–96.9%). Enzymatic digestibility of cellulose-enriched residues was significantly enhanced to 94.0% and was approximately three-fold higher than the untreated material at an enzyme loading of 15 FPU g⁻¹ substrate. The efficient conversion was mainly ascribed to the synergetic and efficient removal of xylan (80.8%) and lignin (63.4%), which resulted in the incompact structure and corrosive surfaces of treated corncobs. Furthermore, DESs could be recycled and reused five times without significant loss of pretreatment performance. The recovered lignin showed a typical structure of HGS, and had high purity (>96%), medium molecular weight (3840–10 900 g mol⁻¹), low polydispersity (1.44–1.97) and good solubility in many organic solvents. Qualitative and quantitative structural characteristics elucidated lignin profiles and their potentials for efficient conversion into value-added products. Overall, technically feasible and significantly efficient DES pretreatments with a shorter pretreatment time provided comprehensive information for effective fractionation and valorization of biomass components for future industrialized processing.