The origin of the bioaccumulative neurotoxin methylmercury (MeHg) in the ocean remains elusive. The current paradigm suggests that microbial methylation of inorganic Hg within the oceanic water column produces monomethylmercury (MMHg) and potentially dimethylmercury (DMHg). Reaction rates and main drivers governing MeHg levels (sum of MMHg and DMHg) are poorly constrained. We conducted ambient Hg species measurements and enriched isotopic tracer experiments in waters of two contrasting marine environments, the oligotrophic Mediterranean Sea (MED) and the mesotrophic Atlantic Ocean (ATL). Maximum subsurface MeHg levels were ~2 times higher in the MED compared to the ATL, essentially driven by higher DMHg concentrations (0.45 ± 0.06 vs 0.16 ± 0.02 pM). Methylation was only detectable in unfiltered subsurface waters and presumably biotically driven. The highest methylation rate (MMHg to DMHg) was observed in subsurface MED waters while reduction and demethylation rates were highest in surface waters of both environments. Experimental reaction rates and the potential microbial activity (based on 16S rDNA) aligned with ambient Hg species distributions. Assuming high DMHg stability and applying our fast experimental DMHg formation rates, a newly developed 1D water column model (MED) successfully reproduced MeHg species distribution, suggesting DMHg plays a key role in the global marine Hg cycle.