We present a new dataset for whole-rock major, trace, isotopic, and phenocryst compositions indicating a genetic link between andesites of the Holocene eruptions of the Bezymianny stratovolcano (the Bezymianny stage), the andesitic to dacitic Late Pleistocene lava dome complex (the pre-Bezymianny stage), and the magnesian to high-alumina basalts of the adjacent Kliuchevskoi Volcano. We demonstrate that volcanic products from the Bezymianny stage of volcano evolution are most likely the products of magma mixing between silicic products of the earliest stages of magma fractionation and the less evolved basaltic andesite parental melts periodically injected into the magma reservoir. In contrast, the intermediate and silicic magmas of the pre-Bezymianny stage together with basalts from Kliuchevskoi much more closely resemble the liquid line of descent and may represent a unique prolonged and continuous calc-alkaline trend of magma evolution from high-magnesian basalt to dacite. As a result of the geothermobarometry, we recognize variable conditions of magma fractionation and magma storage beneath Bezymianny for different magma types during its evolution since the Late Pleistocene: (1) 1100–1150 °C, 500–640 MPa, 1–2.5 wt. H2O for parental basaltic andesite; (2) 1130–1050 °C, 700–600 MPa, 2.5–5 wt. H2O for two-pyroxene andesites; (3) 1040–990 °C, 560–470 MPa, 5–6.5 wt. H2O for orthopyroxene-bearing andesites; (4) 950–1000 °C, 450–150 MPa, 3.5–5.5 wt. H2O for hornblende-bearing andesites; and (5) 950–900 °C, 410–250 MPa, 6–7 wt. H2O for dacites. Repeated basalt injections and magma fractionation combined with internal mixing in the magma chamber are the main processes responsible for both the complex petrography and the geochemical trends observed in the lavas of Bezymianny Volcano.