The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in ³He/⁴He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5 R_A. Potential sources of the ³He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a ³He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 R_A), the persistence of relatively high values (>5 R_A) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased ³He/⁴He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in ³He/⁴ He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of ³He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.