Makushin Volcano on Unalaska Island, Alaska, threatens the Aleutian's largest population centers (Unalaska and Dutch Harbor), yet its eruption mechanisms are poorly known. This study presents a detailed stratigraphic and geochemical investigation of Makushin's most recent highly explosive event: the ca. 8.5 ka Driftwood Pumice eruption. The Driftwood Pumice has measured thicknesses of over 2.5 m, and isopach reconstructions estimate a total deposit volume of 0.3 to 1.6 km3, indicating a VEI 4–5 eruption. Proximal deposits consist of normally-graded, tan, dacitic to andesitic pumice, capped by a thinner dark layer of lower-silica andesitic scoria mixed with abundant lithic fragments. This stratigraphy is interpreted as an initial vent-clearing eruption that strengthened into a climactic ejection of pumice and ash and concluded with vent destabilization and the eruption of somewhat more mafic, gas-poor magma. Within the pumice, geochemical trends, disequilibrium mineral populations, and mineral zonation patterns show evidence of magma mixing between a bulk silicic magma and a mafic melt. Euhedral high-Ca plagioclase (An68–91) and high-Mg olivine (Fo69–77) phenocrysts are in disequilibrium with trachydacitic glass (65–68 wt% SiO2) and more abundant sodic plagioclase (An34–55), indicating the former originally crystallized in a more mafic melt. Tephra whole rock compositions become more mafic upwards through the deposit, ranging from a basal low-silica dacite to an andesite (total range: 60.8–63.3 wt% SiO2). Collectively, these compositional variations suggest magma mixing in the Driftwood Pumice (DWP) magma reservoir, with a systematic increase in the amount of a mafic component (up to 25%) upward through the deposit. Olivine-liquid and liquid-only thermometry indicate the mafic magma intruded at temperatures ~140–200 °C hotter than the silicic magma. Diffusion rates calculated for 5–7 μm thick, lower-Mg rims on the olivine phenocrysts (Fo60 rim vs Fo76 bulk) suggest that the eruption occurred several days to weeks following the mafic injection into a dacitic reservoir. Based on this timing, we infer that the mafic intrusion provided a thermal pulse that initiated convection and volatile exsolution, and ultimately resulted in the DWP eruption. Unalaska's Holocene stratigraphy includes multiple light-dark ashfall couplets with physical and geochemical similarities to the DWP, suggesting that magma mixing may be a common eruptive trigger at Makushin Volcano. •The 8.5 ka Driftwood Pumice is the most recent large eruption of Makushin Volcano.•The first detailed stratigraphy, geochemistry, and mineralogy of a Makushin eruption.•Petrology indicates eruption triggering due to a basalt intruding a dacitic reservoir.•Diffusion modeling shows magma mixing occurred days to weeks prior to the eruption.•Magma mixing may be the main triggering mechanism during Makushin's Holocene history.