Mercury (Hg) is a volatile, bioaccumulative, and toxic heavy metal, and its global distribution is controlled by the Hg biogeochemical cycle in the atmosphere-land-ocean systems and the deep Hg cycle in interior reservoirs (e.g., mantle and crust). The biogeochemical cycle has been relatively well studied, but the deep Hg cycle remains relatively poorly constrained. Mercury isotopes undergo mass-dependent fractionation (MDF) and unique mass-independent fractionation (MIF) which can provide good constraints on large-scale Hg cycling. In this review, we provide a summary of available results on Hg abundance and isotopic composition in the atmosphere-land-ocean systems and interior reservoirs, with a focus on linking the Hg biogeochemical cycle to the deep Hg cycle. Through this effort, a few key points can be pointed out: (1) Natural and anthropogenic activities release large amounts of Hg into the atmosphere, which is transported on a global scale and deposited in terrestrial and marine systems; (2) Major constituents of the mantle and crust, e.g., mid-ocean ridge basalts (MORBs) and granites, show much lower Hg abundance than the atmosphere-land-ocean systems due to volcanic Hg(0) degassing and the formation of Hg-bearing ore deposits; (3) Mercury isotopes, especially Δ199Hg values, are useful in tracing surface Hg recycling into mantle and crust; (4) Hg(II) photo-reduction in the atmosphere yields negative Δ199Hg values in gaseous Hg(0) and positive Δ199Hg values in Hg(II) species, which results in negative Δ199Hg values in terrestrial systems (dominant deposition of Hg(0)) and positive Δ199Hg values in marine systems (dominant deposition of Hg(II); (5) MORBs and arc-related basalts (IABs) show positive Δ199Hg values, suggesting marine Hg recycling into the oceanic crust and upper mantle via plate subduction. Oceanic island basalts (OIBs) and continental flood basalts (CFBs) mostly display near-zero Δ199Hg values, suggesting limited surface Hg recycling into the lower mantle. Granites show positive to negative Δ199Hg values, suggesting the continental crust receives Hg from the metasomatized mantle and remelted terrestrial material. Opposing Δ199Hg values in arc-related and intracontinental hydrothermal systems highlights the great potential of using Hg isotopes for metallogenic tracing.