The vertical distribution of mercury (Hg) was determined at coastal and open ocean sites in the northwest Atlantic and Pacific Oceans. Reliable and diagnostic Hg distributions were obtained, permitting major processes governing the marine biogeochemistry of Hg to be identified. The northwest Atlantic near Bermuda showed surface water Hg concentrations near 4 pM, a maximum of 10 pM within the main thermocline, and concentrations less than or equal to surface water values below the depth of the maximum. The maximum appears to result from lateral transport of Hg enriched waters from higher latitudes. In the central North Pacific, surface waters (to 940 m) were slightly elevated (1.9 ± 0.7 pM) compared to deeper waters (1.4 ± 0.4 pM), but no thermocline Hg maximum was observed. At similar depths, Hg concentrations near Bermuda were elevated compared to the central North Pacific Ocean. This inter-ocean pattern results from a combination of enhanced supply of Hg to the northwest Atlantic by rainfall and scavenging removal along deep water circulation pathways. An upper water column Hg maximum was also observed at a Slope Water station in the northwest Atlantic and in the Tasman Sea. We hypothesize that the source of this Hg comes from diagenetic reactions in oxic margin sediments, releasing dissolved Hg to overlying water. In contrast, a broad minimum (1.5 ± 0.2 pM) in the vertical distribution was observed in the coastal waters off Peru which coincided with the intense oxygen minimum zone in the water column. Removal of dissolved Hg from solution during contact with highly anoxic shelf and slope sediments is proposed as the mechanism for producing this unique profile. Geochemical steady-state box modeling arguments predict a relatively short (~350 years) mean residence time for Hg in the oceans, demonstrating the reactive nature of Hg in seawater and precluding significant involvement in “nutrient-type” recycling. Mercury's distributional features and reactive nature suggest that interaction of Hg with settling particulate matter and margin sediments play important roles in regulating oceanic Hg concentrations. Oceanic Hg distributions are governed by an “external cycling process”, in which water column distributions reflect a rapid competition between the magnitude of the input source and the intensity of the (water column) removal process. In this regard, Hg shares many distributional features with other elements, such as Pb and Bi, which have very short oceanic residence times.