In this study, in-situ synthesis of tungsten boride-carbide composite powders using a mechanochemical processing (MCP) method was investigated on the WO3-B2O3-Mg-C quaternary powder system. The raw blends of WO3-B2O3-Mg-C were processed in a high-energy ball mill for between 8 and 24 h. In addition to the MCP duration, excess B2O3 or C reactants and ball-to-powder weight ratio (BPR) were tested as important process parameters which affected the resultant phases. After the mechanochemical reaction, the products were purified using 6 M HCl solution to eliminate the MgO by-product. FactSage 7.1 and HSC Chemistry Ver.4.1 thermochemical software was used to predict the thermodynamically possible reactions and products. Microstructural properties of the fabricated powders were inspected through X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and energy dispersive spectroscopy (EDS) techniques. Additionally, changes in particle size with duration of the MCP were determined for the synthesized powders. According to the XRD results, tungsten boride and tungsten carbide phases were obtained as the main reaction products for all reaction durations and reactant stoichiometries. Distribution of the total amounts of tungsten boride and tungsten carbide phases were determined respectively as 79.8 and 20.2 wt% for the 8 h-processed stoichiometric powders. However, the total tungsten boride amount decreased with the increasing MCP duration and its respective percentage was calculated as 57.1 wt% for the 24 h-processed stoichiometric powders. In addition, by increasing the processing duration to over 12 h, W2B, W2C and B4C phases were detected. When 200 wt% B2O3 was added, the B:W ratio and total amount of tungsten boride phases in the synthesized powders increased. Similarly, by utilizing 200 wt% C, a significant increase in the B4C peaks together with W2C peaks was detected. Therefore, the properties of tungsten boride-carbide composite powders synthesized by the mechanochemical route are highly affected by the process parameters.