The mechanically triggered interactions in the Ti–C and Hf–C systems performed in a high-energy planetary ball mill are studied. The formation of carbides (TiC and HfC) in the course of mechanochemical synthesis at the different stages is followed by both conventional and synchrotron X-ray diffraction analyses and high-resolution scanning and transmission electron microscopy. The mechanism of the mechanochemically induced reactions in the Ti–C and Hf–C systems leading to the relatively rapid formation of TiC and HfC, respectively, is proposed. The interaction between metallic Ti (Hf) and C proceeds through the formation of mechanocomposites Ti/C (Hf/C) at the first stage, followed by the melting of titanium (hafnium), the spreading it over the carbon particle surface and the crystallization of titanium (hafnium) carbides. It is demonstrated that the mechanosynthesis of ultra-fine TiC and HfC is completed after 20 and 8 min of mechanical activation, respectively.