Hydrothermal liquefaction (HTL) of biomass for biocrude production constitutes an imperative domain within the renewable energy sector, and the co-liquefaction of different types of biomasses (co-HTL) represents a promising strategy for its potential to minimize logistics expenses and maximize synergistic enhancement in biocrude yield and quality. The investigation of co-HTL of carbohydrate-rich, protein-rich and ash-rich feedstocks has unearthed complex interactions among single components of biomass, which result in varying degree of synergistic and antagonistic effects. These co-liquefaction effects dependent on factors such as mixing ratios and temperature, demand a comprehensive understanding of underlying mechanisms. The exploration of co-HTL of biomass single model components (e.g., protein, carbohydrate, lipid) has facilitated advanced insights into these mechanisms. Maillard reaction between protein and carbohydrate, and amidation between protein and lipid, have been extensively observed. Further research efforts are needed to examine carbohydrate and lipid interactions and the variation induced by ash presence. The co-HTL of biomass with various types of plastics has recently shown promising results. Synergistic effects on oil yield and improvements in oil quality have been observed, even though additive effects have been noted as well. There is a necessity for additional research endeavors to investigate the co-HTL of biomass model components in conjunction with plastics and their corresponding monomers, with the aim to elucidate the underlying mechanism more explicitly and ultimately achieve more efficient feedstock selection and process design. These combined findings present co-HTL as the vital areas for future HTL research, reflecting not only its complexity but also its substantial promise in renewable energy and waste management advancements.