Abundant geochemical and petrological data are available for the 1980–1986 and 2004–2006 eruptive periods and some of the units of the most recent Spirit Lake stage at Mount St. Helens. However, with the exception of bulk rock data, few geochemical data are available for the earlier eruptive units, including the Ape Canyon, Cougar, and Swift Creek stages. Here, the early magmatic history of Mount St. Helens has been constrained based on chemical zonation of phenocrysts from the different eruptive units and compositions of melt inclusions determined by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICPMS). Mount St. Helens has erupted adakitic composition magma over its entire 300 000 year history, beyond the most recent Spirit Lake stage as previously identified. The observed geochemical variations are consistent with fractional crystallization and do not require assimilation‐fractional crystallization to produce the observed trends. Similarly, large‐scale mixing of compositionally different magmas is not required. The source of the intermediate to felsic magmas at Mount St. Helens appears to be metamorphosed underplated arc gabbro and geochemical trends suggest that the contribution of underplated material to Mount St. Helens magma has decreased in the modern period.