The interphase region between the carbon monofilament and epoxy matrix in Carbon Fiber Reinforced Polymer Composites (CFRPs) is immensely vital to transfer stress between carbon monofilament and bulk matrix material. To the best of the authors' knowledge, no other research group has studied the interphase region at subsurface and at-depth levels on hygrothermal aged CFRPs. The composite samples were exposed to 60° C and 90% humidity for one and two years. Moisture absorptions were measured periodically to assess water gain in the material. The advanced Atomic Force Microscopy (AFM) based Peak Force Quantitative Nanomechanics Mapping Technique was used to study the physics of the interphase. PFQNM allows non-destructive and simultaneous capture of imaging and mechanical property data with nanometer resolution. The interphase thickness was increased with increased hygrothermal exposure time. The interphase surrounding carbon monofilaments exhibited nonuniform thickness, ranging from ∼85 to 95 nm in the subsurface level, and from ∼10 to 75 nm on the at-depth level. Aged samples showed a decrease in average surface roughness, likely due to swelling of the epoxy matrix caused by the moisture absorption. The water diffusion generally followed Fickian.