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Methylmercury (MeHg) production is controlled by the bioavailability of inorganic divalent mercury (Hg(II)_i) and Hg‐methylation capacity of the microbial community (conferred by thehgcABgene cluster). However, the relative importance of these factors and their interaction in the environment remain poorly understood. Here, metagenomic sequencing and a full‐factorial MeHg formation experiment were conducted across a wetland sulfate gradient with different microbial communities and pore water chemistries. From this experiment, the relative importance of each factor on MeHg formation was isolated. Hg(II)_ibioavailability correlated with the dissolved organic matter composition, while the microbial Hg‐methylation capacity correlated with the abundance ofhgcAgenes. MeHg formation responded synergistically to both factors. Notably,hgcAsequences were from diverse taxonomic groups, none of which contained genes for dissimilatory sulfate reduction. This work expands our understanding of the geochemical and microbial constraints on MeHg formation in situ and provides an experimental framework for further mechanistic studies.