The volumes, melt fractions, and geometries of shallow magma reservoirs strongly influence the hazards associated with volcanic regions. This study investigates these properties of the shallow Mount St. Helens (Washington State, USA) magma reservoir using modeled P-wave velocities (Vp) from data collected during the Imaging Magma Under St. Helens (iMUSH) active-source seismic experiment. This model shows a low Vp anomaly between 3.5 and 14 km depth that is inferred to represent the magma reservoir that feeds this volcano. The shape of this feature is complex compared to previous models of the magma reservoir and at most depths has a central location that is offset to the south/southwest from the volcanic edifice. Using a calculated relationship between Vp and melt fraction, it is estimated that anomalously high melt fractions of 10–12% occur between 4 and 6 km depth within the inferred magma reservoir. A majority of the magma source depths of major eruptions at Mount St. Helens over the past 4 k.y. fall within this same depth range. This relationship suggests persistent properties of the magma reservoir that focus magma accumulation within this depth range over multiple eruptive episodes. Future monitoring of changes in the seismic properties in this shallow portion of the reservoir may improve forecasting for major volcanic eruptions at Mount St. Helens.