A new process for the torrefaction of mixtures of biomass and oily sludges is proposed. A fossil fuel (gasoline or diesel) was added to mixtures of biomass and hydrocarbon-rich sludge (10:1), and this mixture was subject to distillation until all liquids are recovered. The fossil fuel partially dissolved the sludge components and promoted their evenly distribution over the biomass particles. During distillation, the fossil fuel and all the distillable components present in the biomass and sludge were collected and the mixture was subject to temperatures at which a considerable transformation of the non-distillable fraction occurred. The biomass lost water and suffered partial decomposition and rearrangement to yield biochars with HHV of 23.6 and 33.2 MJ/kg. The original biomass had a HHV of 17.9 MJ/kg, but the hydrothermal torrefaction process as well as the fortification with heavy hydrocarbons from the oily sludge significantly increased its calorific value. The distillable liquids were recovered in the form of two immiscible liquid phases: (a) an organic phase mainly composed by the added fossil fuel but also containing the nonpolar volatile components present in the sludge and the biomass, and (b) an aqueous phase that contained the free and emulsified water present in the biomass and in the sludge, but also the water and polar organic components released from the mixture during this thermal treatment. This process takes place at atmospheric pressure and temperatures lower than 300 °C and can be applied to different sludges with high carbon content to promote their energetic valorization.