The present work investigates Ni–nanodiamond and Ni–graphite composites produced by mechanical synthesis and subsequent heattreatments. Processing of nickel–carbon nanocomposites by this powder metallurgy route poses specific challenges, as carbon phases areprone to carbide conversion and amorphization. The processing window for carbide prevention has been established through X-ray diffractionby a systematic variation of the milling parameters. Transmission electron microscopy confirmed the absence of carbide andshowed homogeneous particle distributions, as well as intimate bonding between the metallic matrix and the carbon phases. Ring diffractionpatterns of chemically extracted carbon phases demonstrated that milled nanodiamond preserved crystallinity, while an essentiallyamorphous nature could be inferred for milled graphite. Raman spectra confirmed that nanodiamond particles remained largelyunaffected by mechanical synthesis, whereas the bands of milled graphite were significantly changed into the typical amorphous carbonfingerprint. The results on the annealed nanocomposites showed that milling with Ni accelerated graphitization of the carbon phasesduring heat treatments at 973 and 1073 K in both composites. At the finer scales, the nanocomposites exhibited a remarkable microhardnessenhancement (70%) compared with pure nanostructured nickel. The Hall–Petch relation and the Orowan–Ashby equation areused to discuss strengthening mechanisms and the load transfer ability to the reinforcing particles.