Lassen Peak is a large volcanic dome of late Pleistocene age at the southern end of the Cascades arc. This dacitic dome has a complex phenocryst assemblage of mixed-magma origin and has been effectively dated for the first time using the 40Ar/39Ar technique. Emplacement of the dome occurred after an advance of late Pleistocene glaciers from an older volcanic cone to the southwest and from a large volcanic plateau to the east. Emplacement of the Lassen Peak dome before the latest major Pleistocene glaciation—correlated by previous workers with the Tioga glaciation of the Sierra Nevada—formed a high peak that intercepted some of the precipitation that previously had fed the plateau ice cap, forming a 10-km-long glacier from a cirque on the northeast side of the peak. The weighted mean 40Ar/39Ar age of the dacite of Lassen Peak is 28.3 ± 2.7 ka. Comparison of its remanent magnetic direction to that of sedimentary deposits at Mono Lake, California, indicates a correlation to strata of 27 ± 1 ka. The dacite of Kings Creek, which erupted from the site of Lassen Peak before a glacial advance that predated the Lassen Peak dome, overlies peat that has a U-Th-calibrated 14C age of 37.6 ± 0.2 ka. The dacite of Kings Creek yielded a 40Ar/39Ar age of 32 ± 17 ka. The paleomagnetism of this dacite is similar to that of Mono Lake sediments of 35 ± 1 ka age. Thus, a late Pleistocene glacial advance probably began in the Lassen region between about 35 and 27 ka.