Methylmercury (MeHg) is a globally pervasive contaminant that biomagnifies in food webs and can reach toxic concentrations in consumers at higher trophic levels, including wildlife and humans. The production of MeHg, and its subsequent entry and biomagnification in food webs, is governed by a complex suite of biogeochemical, physical, and ecological processes, resulting in spatial variation in the distribution of MeHg. To better understand the link between MeHg production in sediments and MeHg bioaccumulation in biota, we evaluated the effects of habitat biogeochemistry, food web structure, and diet composition on bioaccumulation in the wetland-obligate California black rail (Laterallus jamaicensis coturniculus) at three wetlands along the Petaluma River in northern San Francisco Bay, California, USA. We also characterized meso-scale spatial variation by assessing differences among marsh subhabitats in sediment biogeochemistry and MeHg concentrations of sediments, surface waters, and tissues of biota.