The research and development into imidazolium salts (IMSs) and their derivatives (N-heterocyclic carbene, NHCs) for sustainability research in biomass conversion and CO2 capture/utilization is reviewed. Research into using IMSs/NHCs in green synthesis, separation, and green materials has increased steadily over the past few years. Recent work utilizing imidazolium ionic liquids in dissolving and converting biomass are assessed. IMSs are particularly useful for directly processing raw biomass, such as cellulose, as feedstock to produce 5-hydroxymethylfurfural (HMF) under mild conditions. Catalytic activities of N-heterocyclic carbenes in sugar conversion are also studied, with a view towards practical industrialization. Imidazolium ionic liquids for CO2 capture and CO2 utilizations are also assessed. Imidazolium salts (IMSs) and their derivatives provide a promising way for carbon dioxide capture and utilization.