The Proterozoic Eon (2500–542 Ma) appears to have been a warm period bookended by glaciations, despite a 5%–18% reduction in solar output compared to modern during this interval. Radiative-convective climate models suggest that glaciation could have been avoided if pCO2 were 30–300× preindustrial atmospheric levels (PIAL, 280 ppmv). Constraints from late Mesoproterozoic (ca. 1.2–1.0 Ga) microfossil calcification sheaths and paleosol mass balance, however, suggest that pCO2 may have been no higher than 10× PIAL. In the lower oxygen Mesoproterozoic atmosphere, an increased CH4 flux from methanogenic bacteria may have contributed additional greenhouse warming. We use a fully coupled atmosphere-ocean general circulation model (the U.S. National Center for Atmospheric Research Community Earth System Model, CESM) to test whether these pCO2 constraints are consistent with the absence of widespread glaciation inferred from the geologic record. We vary pCO2 and pCH4 between 1400 and 2800 ppmv and 3.5 and 140 ppmv, respectively, using a reconstructed 1.0 Ga paleogeography and solar output reduced by 9%. Our simulations suggest that ice-free conditions can be maintained at 10× PIAL CO2 when CH4 is 140 ppmv. When CH4 is lowered to 28 ppmv at 10× PIAL CO2, or if pCO2 is lowered to 5× PIAL, permanent land snow cover at high and middle latitudes suggests that glaciation would be more extensive than preindustrial conditions, but with warm tropical regions. Global glaciation occurs if pCO2 is reduced below 5× PIAL. Overall, our simulations suggest that an ice-free climate for the Mesoproterozoic (1.6–1.0 Ga) is consistent with the relatively low pCO2 implied from proxies if CH4 or other greenhouse gas concentrations were sufficiently elevated.