Large-scale biorefineries converting lignocellulosic biomass into chemicals, fuels, and energy require a cost-effective pretreatment process that can effectively fractionate all main lignocellulose constituents. A mild organosolv process has been developed using a system of dimethyl carbonate (DMC) and ethylene glycol (EG) as solvent. Softwood biomass (pine, spruce, cedar, and Douglas fir) was fractionated using mild conditions: 140 °C, 40 min, DMC-EG, and sulfuric acid. Organosolv pulping of the softwood biomass usually leads to poor delignification hampering enzymatic cellulose hydrolysis. However, for the developed system, effective pretreatment and subsequent enzymatic cellulose hydrolysis into glucose (up to 84.7%) was observed in combination with a high yield of monomeric hemicellulose sugars and monophenolic compounds from lignin (up to 98% compared to theoretical monomer yield). In sum, effective fractionation and in situ lignin depolymerization was demonstrated for various softwood feedstock combined with limited solvent loss at mild conditions and low reactor pressure.