We examined microbial methylmercury (MeHg) degradation in sediment of the Florida Everglades, Carson River (NV), and San Carlos Creek (CA), three freshwater environments that differ in the extent and type of mercury contamination and sediment biogeochemistry. Degradation rate constant (k_deg) values increased with total mercury (Hg_t) contamination both among and within ecosystems. The highest k_deg's (2.8−5.8 d^-1) were observed in San Carlos Creek, at acid mine drainage impacted sites immediately downstream of the former New Idria mercury mine, where Hg_t ranged from 4.5 to 21.3 ppm (dry wt). A reductive degradation pathway (presumably mer-detoxification) dominated degradation at these sites, as indicated by the nearly exclusive production of ¹⁴CH₄ from ¹⁴C-MeHg, under both aerobic and anaerobic conditions. At the upstream control site, and in the less contaminated ecosystems (e.g. the Everglades), k_deg's were low (≤0.2 d^-1) and oxidative demethylation (OD) dominated degradation, as evident from ¹⁴CO₂production. k_deg increased with microbial CH₄ production, organic content, and reduced sulfur in the Carson River system and increased with decreasing pH in San Carlos Creek. OD associated CO₂ production increased with pore-water SO₄^2- in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparison indicates that severely contaminated sediments tend to have microbial populations that actively degrade MeHg via mer-detoxification, whereas OD occurs in heavily contaminated sediments as well but dominates in those less contaminated.