Supersaturation is the fundamental parameter driving crystal formation, yet its dynamics in the growth of colloidal nanocrystals (NCs) remain poorly understood. Here, we demonstrate an approach to characterize supersaturation during classical NC growth. We develop a framework that relates noninvasive measurements of the temporal, size-dependent optical properties of growing NCs to the supersaturation dynamics underlying their growth. Using this approach, we investigate the seed-mediated growth of colloidal Au nanocubes, identifying a triphasic sequence of supersaturation dynamics: rapid monomer consumption, sustained supersaturation, and then gradual monomer depletion. These NCs undergo different shape evolutions in different phases of the supersaturation dynamics. As shown with the Au nanocubes, elucidated supersaturation profiles enable the prediction of growth profiles of NCs. We then apply these insights to rationally modulate NC shape evolutions, decreasing the yield of impurity products. Our findings reveal that the supersaturation dynamics of NC growth can be more complex than previously understood. As our approach is applicable to many types of NCs undergoing classical growth, this work presents an initial step towards more deeply interpreting the phenomena governing nanoscale crystal growth and provides insight for the rational design of NCs.