Methanogenic archaea are pivotal drivers of carbon cycling in anoxic environments. Growing evidence shows that they also participate in the biogeochemical cycling of metal(loid)s, yet the underlying transformation mechanisms have not been systematically summarized. This review integrates the latest findings to dissect how methanogenic archaea oxidize, reduce, methylate, and demethylate representative metal(loid)s, including iron (Fe), mercury (Hg), vanadium (V), chromium (Cr), cadmium (Cd), arsenic (As), and selenium (Se). The research findings are summarized as follows: (1) Fe(Ⅲ) reduction exerts bidirectional control over methanogenesis. When extracellular Fe(Ⅲ) reduction is not coupled to energy metabolism, it markedly suppresses the growth and methane production of methanogenic archaea (e.g., Methanosarcina barkeri). Conversely, when extracellular Fe(Ⅲ) reduction is coupled to energy metabolism, it stimulates the physiological and metabolic activities of methanogenic archaea (e.g., Methanosarcina acetivorans). (2) For mercury methylation, methanogenic archaea convert Hg(Ⅱ) to methylmercury (MeHg) via a methyltransferase encoded by the hgcAB gene cluster. In some species (e.g., Methanomassiliicoccus luminyensis), the observed methylation activity is associated with enzymes released from lysed cells. (3) Arsenic transformation runs with diverse mechanisms. Methanosarcina acetivorans methylates As(Ⅲ) via the arsenic methyltransferase (ArsM) and concurrently reduces As(V) to As(Ⅲ) through arsenate reductase (ArsC), whereas archaeal communities in paddy soils are capable of demethylating organic arsine. (4) Selenium biotransformation exhibits dual effects: low concentrations of selenium nanoparticles (SeNPs) enhance methanogenic activity and induce organoselenium synthesis, whereas high concentrations trigger oxidative stress. Environmentally, metal (loid)s markedly affect the metabolic activity and community structure of methanogenic archaea by altering redox potential, competing for electron acceptors, or imposing toxic stress. This review highlights the multifunctionality of methanogenic archaea in metal (loid) cycling and proposes that future work should combine meta-omics and metabolomics approaches to elucidate enzyme-level mechanisms, while exploring methanogenic archaea-based strategies for the bioremediation of metal (loid) contamination.