The mixed carbide Ti-Zr-C has been synthesized through carbothermal reduction of TiZrO4 at 2200 °C, 2300 °C, and 2400 °C. As-synthesized carbide was subsequently aged at 1400 °C to study phase separation. Microstructural investigations and nano-indentation measurements were performed. It was found that the synthesis temperature was important for the homogeneity and porosity of the as-synthesized powder, which strongly influenced the subsequent phase separation upon aging. The phase separation occurred via discontinuous precipitation, and high-angle boundaries were preferred. Furthermore, the characteristics of discontinuous precipitation in the present Ti-Zr-C system with a miscibility gap were compared to those of classical discontinuous precipitation. Finally, fully decomposed particles were found to be slightly harder than the unaged carbide particles.