The hydrogen environment-assisted cracking (HEAC) susceptibility of a precipitation-hardened Ni-based superalloy was evaluated via slow-rising stress intensity loading while immersed in NaCl solution and exposed to an applied potential of -1200 mVSCE. Three heat treatment conditions, corresponding to the under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the over-aged condition. Variables which are expected to vary under this isothermal heat treatment are grain size, grain boundary impurity concentration, and precipitate morphology. A detailed assessment of grain size and grain boundary impurity concentration evolution with aging time revealed a minimal influence of these variables on the measured HEAC susceptibility of Monel K-500. Transmission electron microscopy revealed a distinct shift from planar to wavy slip between the peak-aged and over-aged conditions, suggesting that the presence of planar slip contributes to HEAC susceptibility. A brief analysis of the mechanistic implications of this finding is presented and the need for additional experiments which examine crack tip deformation processes is discussed.