: In this study, isotopic compositions of atmospheric total gaseous mercury (TGM) were measured in the Mt. Changbai (MCB) temperate deciduous forest and the Mt. Ailao (MAL) subtropical evergreen forest over a 1- year period. Higher δ202HgTGM values were observed under the forest canopy than above the forest canopy in the MCB forest. The vertical gradients in δ202HgTGM and Δ199HgTGM are positively correlated with the satellite-based normalized difference vegetation index (NDVI, representing the vegetation photosynthetic activity), suggesting that a strong vegetation activity (high NDVI) induces both mass-dependent and mass-independent fractionation of TGM isotopes. The observed δ202HgTGM and Δ199HgTGM showed seasonal variations. Mean δ202HgTGM and Δ199HgTGM in summer were 0.35−0.99‰ and 0.06−0.09‰ higher than those in other seasons in the MCB forest. In contrast, the highest seasonal δ202HgTGM in the MAL forest was observed in winter at 0.07−0.40‰ higher than the values found in other seasons. The variability of δ202HgTGM and Δ199HgTGM in MCB was attributed to vegetation activities, whereas the seasonal δ202HgTGM in the MAL forest was driven by the exposure of air masses to anthropogenic emissions. Using the data in this study and in the literature, we concluded that vegetation activity and anthropogenic Hg release are the main drivers for the spatial variations in TGM isotopic compositions in the northern hemisphere.