Many studies provide evidence relating lower human arsenic (As) methylation efficiency, represented by high % urinary monomethylarsonic acid (MMA(V)), with several arsenic-induced diseases, possibly due to the fact that MMA(V) serves as a proxy for MMA(III), the most toxic arsenic metabolite. Some epidemiological studies have suggested that indigenous Americans (AME) methylate As more efficiently, however data supporting this have been equivocal. The aim of this study was to characterize the association between AME ancestry and arsenic methylation efficiency using a panel of ancestry informative genetic markers to determine individual ancestry proportions in an admixed population (composed of two or more isolated ancestral populations) of 746 individuals environmentally exposed to arsenic in northwest Mexico. Total urinary As (TAs) mean and range were 170.4 and 2.3–1053.5 μg/L, while %AME mean and range were 72.4 and 23–100. Adjusted (gender, age, AS3MT 7388/M287T haplotypes, body mass index (BMI), and TAs) multiple regression model showed that higher AME ancestry is associated with lower %uMMA excretion in this population (p <0.01). The data also showed a significant interaction between BMI and gender indicating negative association between BMI and %uMMA, stronger in women than men (p <0.01). Moreover age and the AS3MT variants 7388 (intronic) and M287T (non-synonymous) were also significantly associated with As methylation efficiency (p = 0.01). This study highlights the importance of BMI and indigenous American ancestry in some of the observed variability in As methylation efficiency, underscoring the need to be considered in epidemiology studies, particularly those carried out in admixed populations.