Mechanical alloying in a high-carbon medium (petroleum) is used to fabricate a W_1.5FeCoNi medium-entropy alloy (MEA) in situ reinforced by tungsten carbide (WC). During high-energy ball milling of a mixture of elemental metal powders (W, Fe, Co and Ni) for 20 hours, a b.c.c. solid solution and dispersed WC particles with a grain size from 0.5 μm to 1 μm are formed. The carbide-phase formation involves the decomposition of petrol, the generation of carbon radicals, and a subsequent reaction with tungsten that leads to the formation of WC by both direct reaction with W particles and diffusion into the b.c.c. lattice. XRD and SEM analyses reveal gradual alloying, crystallite refinement, and homogenization of a b.c.c. solid-solution matrix. DSC measurements show two endothermic peaks at 885^oC and 1040^oC, corresponding to short-range ordering, sintering of the powder, secondary WC precipitation, and randomization of a solid solution with possible b.c.c.–f.c.c. transformations. The in situ carbide reinforcement and uniform microstructure are expected to enhance significantly the mechanical properties and thermal stability of the MEA, demonstrating the feasibility of this approach for high-performance carbide-strengthened alloys.