In this work, a combination of mechanical and chemical pretreatments using urea on corn residues (leaves and stems) was evaluated to obtain total reducing sugars (TRSs). The residues were characterized via high-performance liquid chromatography (HPLC) to quantify biomass composition. During the mechanical pretreatment, the particle size of the biomass was reduced to 0.5, 1, and 2 mm. The chemical pretreatment was performed with urea solution at different concentrations (2, 5, and 10% w/v) and a fixed biomass-to-solvent ratio of 1:25 (g/mL) as well as stirring at 150 rpm for 20 h. The effect of temperature on the pretreatment results was evaluated by varying such operating variables in 30 and 50 °C. After both pretreatments, hydrolysis was carried out in an autoclave using sulfuric acid at 1% v/v at 121 °C for 1 h. The content of TRS was quantified using 3,5-dinitrosalicylic acid (DNS) method and biomass after pretreatment was characterized via Fourier transform infrared (FT-IR). For both leaves and stems, the HPLC technique reported the presence of 47.4 g of cellulose, 40.04 g of hemicellulose, and 26.38 g of lignin. It was found that the highest amount of TRS (36.50 g/L) was obtained with a urea concentration of 2% at 50 °C using a particle size of 0.5 mm. The production of TRS was significantly higher for pretreated biomass than that for raw corn residues, confirming the importance of both mechanical and chemical pretreatments to reach better delignification results.