The specification discloses a highly macroporous catalyst for hydroprocessing and hydroconversion of heavy hydrocarbon feedstocks. The high macroporosity catalyst incudes an inorganic oxide, molybdenum, and nickel components. It has a pore structure such that at least 18% of its total pore volume is in pores of a diameter greater than 5,000 angstroms and at least 25% of its total pore volume is in pores of a diameter greater than 1,000 angstroms. Preferably, the pore structure is bimodal. The catalyst is made by co-mulling the catalytic components with a high molecular weight polyacrylamide followed by forming the co-mulled mixture into a particle or an extrudate. The particle or extrudate is dried and calcined under controlled calcination temperature conditions to yield a calcined particle or extrudate of the high macroporosity catalyst composition.