Glaciers play a fundamental role in shaping mountainous landscapes. As they retreat and thin, the loss of ice can influence the stability of adjacent rock slopes. However, not all slopes currently undergoing glacier retreat, nor those that were previously covered by glaciers, are susceptible to failure. The impact of glacier retreat and thinning on slope stability is mainly shaped by the structural geological settings and the associated failure mechanisms, though the timing can be coincident or delayed with respect to glacier disappearance. The slopes adjacent to Portage Glacier, Alaska, provide an insightful case study for examining the complex interactions between glacier dynamics and slope (in)stability. This study focuses on two large rock slope instabilities situated above the thinning and retreating Portage Glacier and its associated proglacial lake. To explore the relationship between glacier change and slope deformation, we employed a combination of field observations, remote sensing techniques, and kinematic analysis. Our findings indicate that glacier thinning drives the progressive development of these two rock slope instabilities, with deformations initiating once a critical glacier thickness threshold is exceeded. Near the instabilities, unfavorable structural conditions and kinematic freedom facilitate rock mass movement. However, up-glacier from the two instabilities, the structural conditions become more favorable for stability, with no signs of deep-seated rock slope instabilities currently observed.