The fabrication of dense submicrometer grained B4C–TiB2 composite (molar ratio equal to 1:1) is investigated in this work through the combination of mechanical (Ball Milling) and electric current (spark plasma sintering or SPS) activation of Ti, B, and graphite reactants. The full conversion of 8 h milled powders is obtained at much lower temperature (about 1200°C), as compared with simply blended reactants (>1600°C). The formation of TiB2 generally precedes that of B4C. In addition, regardless of the milling treatment, an increase in the heating rate during SPS is found to produce a transition of the mechanism governing the formation of TiB2 and B4C, from gradual solid–solid to rapid combustion-type behavior. Nevertheless, when unmilled powders are used, not negligible amounts of unreacted and intermediate species are still present in the product resulting after the combustion synthesis event. In contrast, ball milled powders reacted completely under combustion regime, even when relatively milder heating rate conditions are adopted. As a consequence of the mechanical treatment, SPS product density increases from about 82% to 94% of the theoretical value (I = 1100 A). Correspondingly, a material with homogeneous phase distribution and grain size down to 100–200 nm is obtained. In this regard, when a combustion synthesis regime is established, a slight coarsening in product microstructure (1–2 μm) is observed. A further improvement of product density (higher than 96.5%) is obtained, at the expenses of a certain grain growth, when the applied current intensity is augmented to 1200 A.