Nanoscale WC and Mo$_2$C carbides are synthesized from the elemental metal powder (with particle size of about 40 $\mu$m; purity is not less than 99.6% wt.) and the carbon nanotubes (CNTs, with average diameter of 10–20 nm) by mechanical alloying in a high-energy planetary ball mill for the first time. Nature of interaction of the charge components at processing in a ball mill is studied on test samples (selected at each 1–2 hours of synthesis) using a complex of x-ray techniques. These techniques include: a full-profile analysis for the primary processing of diffractograms obtained with DRON-3M apparatus; qualitative and quantitative phase analysis for determining the phase composition of the products of synthesis; x-ray structural analysis to verify and refine the structural models; Williamson–Hall method for determining the grain sizes of synthesized carbides and microdistortions of their crystal lattice. Four hours of charge processing result in a formation of the high-temperature W$_2$C and Mo$_2$C carbides, the crystal structure of which is related to the $\zeta$-Fe$_2$N structure type with vacancies within the metal sublattice. Further milling of W–CNT mixture (up to 10 hours) is accompanied by the W$_2$C + CNT $\to$ WC transformation, while processing of the Mo–CNT mixture leads to its dispersion. The effect of CNTs on mechanochemical synthesis of the WC and Mo$_2$C carbides is considered. As shown, the mechanical alloying of W/Mo–CNT is a highly effective method for the fabrication of the WC and Mo$_2$C carbides. Due to their unique mechanical characteristics (high hardness, wear resistance, and strength), these materials are widely used in making materials for the abrasive and metal cutting tools.