In the present work, brewers’ spent grain (BSG), which represents the major by-product of the brewing industry, was recovered from a regional brewery and fractionated in order to obtain a complete valorization. In particular, the whole process was divided in two main parts. A first pretreatment with hot water in an autoclave allowed the separation of a solution containing the soluble proteins and sugars, which accounted for 25% of the total starting biomass. This first step allowed the preparation of a medium that was successfully employed as a valuable growing medium for different microbial fermentations, leading to valuable fungal biomass as well as triglycerides with a high content of linear or branched fatty acids, depending on the microorganism used. The solid water-insoluble residue was then submitted to a lignocellulose deep eutectic solvent-mediated fractionation, which allowed the recovery of two important main fractions: BSG cellulose and BSG lignin. The latter product was tested as potential precursor for the development of cement water reducers with encouraging results. This combination of treatments of the waste biomass appeared to be a promising sustainable strategy for the development of the full exploitation of BSG from a circular economy perspective.