The origin and nature of the carbon reservoirs in terrestrial and lunar mantles have profound implications for planetary volatile transportations and interior evolution. Both terrestrial and lunar mantles were primitively reducing when they formed, likely by core-mantle differentiation processes early in planetary history. Primitive terrestrial and lunar mantles held > 1000 and > 100 ppm carbon, respectively, mostly as carbide species in metal phases coexisting with silicate minerals. As silicate melting and metal-silicate differentiation proceeded on Earth, carbon was effectively and rapidly partitioned into the silicate melts because of its strong affinity for oxidized species (e.g., carbonate, CO2), leaving behind a carbon-depleted residual mantle and a volatile-rich atmosphere as the Earth's crust formed. In contrast, due to the absence of plate tectonics on the Moon and the limited volatile recycling capability of the Moon's stagnant lid regime, a large fraction of the carbon reservoir, as nano- to micro-sized carbides, was retained within the lunar mantle, without developing a thick volatile-rich atmosphere.