The double-crested cormorant (Phalacrocorax auritus) colony at Little Galloo Island, Lake Ontario has been a Great Lakes focal point of controversy regarding cormorant–fish interactions for over two decades. We examined cormorant diet and fish consumption at the colony from 1992 to 2013. During this time period, two events, management actions and round goby (Neogobius melanostomus) invasion, occurred that affected the number of fish consumed by cormorants and their diet composition. The purpose of this study was to evaluate the effects of round goby on the feeding ecology of cormorants and evaluate the efficacy of management actions on meeting cormorant population targets at the colony. Round goby first appeared in the diet in 2004 (0.8%) and within one year were the primary prey (29.3%). The presence of round goby in the diet of cormorants: (1) eliminated seasonal variability in diet composition, (2) reversed seasonal trends in the number of fish consumed daily, (3) increased daily fish consumption, and (4) significantly reduced the consumption of other species including yellow perch and smallmouth bass. Management actions, such as egg oiling and culling, were also effective in reducing nesting activity and the number of cormorant feeding days at the Little Galloo Island colony. There is evidence that the combination of management actions and round goby may have allowed some population recovery of yellow perch and smallmouth bass in eastern Lake Ontario.