This study empirically investigates the effects of corporate slack resources on two types of innovation investment activities—R&D and advertising—and examines how the ownership–control disparity of controlling shareholders influences these relationships. Using panel data of 1,383 firms belonging to large Korean business groups over a seven-year period, the analysis reveals that the impact of slack resources on innovation investment differs depending on the source of slack. Specifically, R&D investment utilizes both internally reallocated and externally acquired resources, whereas advertising investment depends primarily on readily accessible slack without internal adjustment. The results further demonstrate that both R&D and advertising investments exhibit an inverted U-shaped relationship with slack resources, indicating that there exists an optimal level of slack that maximizes innovation investment efficiency. Moreover, the ownership–control disparity either amplifies or moderates the effect of slack differently across investment types, implying that controlling shareholders significantly influence the optimal level of resource utilization. These findings provide important insights into the investment behavior of firms operating under Korea’s unique pyramidal ownership structure and underscore the need for appropriate resource allocation strategies aligned with the type and timing of future innovation investments.