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Quantifying the buildup of pre-eruptive excess pressure in volcanic systems is crucial for the understanding of eruption-triggering mechanisms. The 2006 eruption of Augustine Volcano, Alaska, provides an excellent opportunity to investigate the response of the crystal cargo within a crystal-rich, intermediate host magma to temperature and pressure perturbations induced by mafic recharge shortly before the eruption. We use compositional changes of plagioclase together with adjoined and concurrently grown orthopyroxene to quantify these changes in magma conditions in response to this recharge event. We find ∼3.9 mol% anorthite increase in plagioclase is best explained by a 7.7 MPa (0.4−37 MPa, 80% uncertainty bounds) increase in magmatic pressure prior to the 2006 eruption. This value is close to the estimated tensile strength of rocks, suggesting that the pressure increase due to mafic recharge was likely sufficient to initiate dike intrusion and possibly trigger the eruption. This approach can be implemented broadly at crystal-rich, intermediate, and hydrous arc volcanoes, can provide independent estimates of pre-eruptive pressure changes to complement geophysical models, and can add new insights into the physical processes that lead to volcanic eruptions.