Expansion of global forest cover via afforestation, reforestation, and forest restoration is a widely proposed nature-based solution for climate change mitigation, yet its effect on wildfire activity is poorly understood. As anthropogenic climate change intensifies and wildfire regimes change globally, evaluating the interactions between forest expansion, climate and population change is critical. We assess how large-scale forest expansion influences future fire activity and the land carbon sink using the Community Land Model version 5 (CLM5) with a mechanistic fire module. We simulate a maximum forest scenario (~ 750 Mha by 2100) under 2 °C and 4 °C warming pathways and compare it to three different land use trajectories with varying levels of forest cover and population change. We find that tropical forest expansion decreases fire activity by halting deforestation fires and replacing flammable grasslands with less flammable tree cover. In contrast, temperate forest expansion, such as in the Mediterranean, central Asia and continental US, can more than double fire carbon emissions under high warming, due to drier conditions and increased fuel loads. Population changes also influence fire regimes, with rising population growth in sub-Saharan Africa suppressing fire and reducing burned area, while decreasing populations in Europe and parts of East Asia are associated with increased fire activity. Finally, fires reduce the global land carbon sink by up to ~ 60 PgC by 2100, equivalent to ~ 5.6 times present-day annual CO₂ emissions, emphasising the need to incorporate fire into climate mitigation planning. Our results suggest that forest expansion can both reduce and intensify fire risk depending on location and that fire-climate-land-human feedbacks must be accounted for in nature-based CO₂ mitigation strategies.