In ecosystems worldwide, the presence of overabundant ungulates (e.g. deer, cows) and the invasion of exotic plants are disrupting native communities. A recent hypothesis causally links these problems implicating overabundant ungulates in enhancing invaders’ demographic success. We tested this hypothesis in a forest where white-tailed deer are over-abundant and garlic mustard is aggressively invading. Using long-term, replicated deer exclusion/deer access plot pairs, we quantified population density, growth, and decline of this invader and native plants. We conclusively demonstrate that deer are required for garlic mustard success; its local extinction is projected where deer are absent. Our findings provide the first definitive support connecting overabundant ungulates to enhanced invader success, with broad implications for bio-diversity and ecosystem function.