Ground thermal conditions in marginal permafrost in Mongolia were assessed using ground temperatures measured year-round at 69 borehole sites. Permafrost is continuous in northern Mongolia and exists as sporadic/isolated patches in the south. Ground temperatures are strongly controlled by local environmental factors, such as topographic depressions that concentrate cold air during winter, ice-rich strata that prevent penetration of sensible heat, and tree cover that reduces incident solar radiation. Permafrost temperatures are typically between -1 and 0 degrees C; colder permafrost (< -2 degrees C) occurs in the northern extent of continuous permafrost and at high elevations in the sporadic/isolated permafrost zones. Relict permafrost, which is thermally disconnected from seasonal air temperature fluctuations, is present near the latitudinal and elevational limits of perennially frozen ground. Cold and thermally responsive permafrost is dominant in the continuous and discontinuous zones, while warm and thermally unresponsive permafrost is dominant in the sporadic and isolated zones. Overall, the climate-driven permafrost in the colder regions is stable, while the ecosystem-driven permafrost in the warmer regions is degrading.