Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic food-webs to levels that are a widespread concern for human and ecological health. Here we investigate the association between dissolved organic matter (DOM) in seawater and Hg speciation and uptake using experimental data and field measurements from Long Island Sound (LIS) and the Northwestern Atlantic continental margin (NWA). We measured differences in DOM composition across sampling stations using excitation emission matrix (EEM) fluorescence spectroscopy and further separated DOM into terrestrial and marine components using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased linearly with a decreasing fraction of fluorescence attributable to DOM components with a terrestrial rather than marine origin. These results are corroborated by experimental data showing higher MeHg uptake by cells in the presence of predominantly marine DOM compared to terrestrial DOM. Highest fractions of dissolved gaseous mercury were also found at stations with the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability in offshore marine waters.