The method of isotope dilution and tracing, based on ethylation-purge and trap-gas chromatography (GC)-inductively coupled plasma mass spectrometry (ICP-MS), has been widely used for the accurate analysis of methylmercury (MeHg) and for tracking its environmental fates (e.g., formation/degradation). However, the tedious ethylation derivatization and purge and trap processes limit analytical throughput. In this work, an automatic ethylation derivatization and purge and trap method, followed by GC separation, pyrolysis, and ICP-MS detection, was developed for MeHg analysis. The throughput and detection limits of this proposed method for MeHg were 7.5 min/sample and 0.03 ng L-1, respectively, with a relative standard deviation of 3.7%. The accuracy of the developed method was validated by the analysis of a spiked water sample and a certified reference material (DORM-4, Fish protein) using isotope dilution (Me201Hg). In addition, anaerobic Geobacter sulfurreducens PCA-mediated methylation of Hg2+ and demethylation of MeHg was monitored by using double-enriched isotope tracing (199Hg2+ and Me201Hg). This automatic ethylation-purge and trap-GC-ICP-MS method is promising for routine MeHg analysis with isotope dilution/tracing.