Gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), particulate-bound mercury (PBM), and the isotopic composition of total gaseous mercury (TGM = GEM + GOM) were measured at the Lulin Atmospheric Background Station (LABS), a mountain background site downwind of the East Asia continent, from February 2015 to January 2016. Mean (±S.D.) concentrations of GEM, GOM, and PBM were 1.54 ± 0.34 ng m⁻³, 14.0 ± 27.8 pg m⁻³, and 2.6 ± 3.7 pg m⁻³, respectively. Concentration-Weighted Trajectory (CWT) analysis indicated that East and coastal China were the main source regions of elevated GEM observed at LABS. Weekly integrated TGM showed significant variations in δ²⁰²Hg_TGM (−0.42 to 0.41‰) and Δ¹⁹⁹Hg_TGM (−0.31 to −0.13‰). Both δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM showed clear seasonal variations, but the seasonal pattern of δ²⁰²Hg_TGM was different from that of Δ¹⁹⁹Hg_TGM. The lowest mean δ²⁰²Hg_TGM was observed in spring (−0.15‰), while the lowest mean Δ¹⁹⁹Hg_TGM was observed in summer (−0.24‰). A significant positive correlation (R² = 0.38, p < 0.01) existed between δ²⁰²Hg_TGM and normalized difference vegetation index (NDVI), suggesting vegetation activity was an important factor controlling the seasonal variation of δ²⁰²Hg_TGM. In contrast, air mass transport path, which influences the source region of TGM to LABS, was likely the driver for the seasonal variations of Δ¹⁹⁹Hg_TGM. Furthermore, results from the air mass classification showed distinguishable Δ¹⁹⁹Hg_TGM signature for different air mass source regions, with air masses from South China Sea and western Pacific Ocean being characterized by more negative Δ¹⁹⁹Hg_TGM (−0.25 ± 0.04‰). Results of this research demonstrated a clear difference in isotopic compositions of atmospheric Hg between continental and marine air masses.