A histogram of 778 isotopic ages of magmatic rocks younger than Eocene in central Mexico shows a multimodal distribution with peaks at about 30 Ma, 23 Ma, 10 Ma, and 5 Ma. The sample suite displays systematic spatial variations with age that likely reflect the protracted transition from the north-northwest–trending arc of the Sierra Madre Occidental to the east-west–trending Mexican Volcanic Belt. The reorientation of the arc is accompanied by a change in the dominant composition of the products from silicic ignimbrites and rhyolites to andesitic and basaltic lavas. The observed transition is related to the Miocene reorganization of the subduction system following the cessation of subduction off Baja California and the eastward motion of the Caribbean–Farallon–North America triple junction along the southeastern margin of Mexico. Our data support an early–middle Miocene age for the initiation of subhorizontal subduction in southern Mexico and confirm that the locus of arc volcanism was primarily controlled by the geometry of plate boundaries and the thermal structure of the subducting slab.