A simple, inexpensive, and modular method to directly illuminate NMR samples for in situ analysis of photochemical transformations is reported. The versatility of this technique is demonstrated by analyzing the light‐induced propagating front for small‐molecule photoswitches and the kinetics of photocontrolled living radical polymerizations. In situ measurements allow oxygen‐sensitive and rapid photoevents to be studied in detail, leading to reliable determination of photoswitching quantum yields and polymerization rates. By systematically tuning light intensity, a direct relationship between propagation rate and intensity is revealed. Of particular note is the facile translation of the conditions identified through this NMR analysis to analogous benchtop experiments with insight into the nature of the photoreactive species.