Recent accumulation of evidence across taxa indicates that the ecological impacts of invasive alien species are predictable from their Functional Response (FR; e.g. the maximum feeding rate) and Functional Response Ratio (FRR; the FR attack rate/handling time ratio). Here, we experimentally derive these metrics to predict the ecological impacts of both juvenile and adult lionfish (Pterois volitans), one of the world’s most damaging invaders, across representative and likely future prey types. Potentially prey-population destabilising Type II FRs were exhibited by both life stages of lionfish towards four prey species: Artemia salina, Gammarus oceanicus, Palaemonetes varians and Nephrops norvegicus. FR magnitudes revealed ontogenetic shifts in lionfish impacts, while lionfish FRR values were substantially higher than mean FRR values across known damaging invasive taxa. Thus, both life stages of lionfish are predicted to contribute to differing but high ecological impacts across prey communities, including commercially important species. With lionfish invasion ranges currently expanding across multiple regions globally, efforts to reduce lionfish numbers and population size structure, and provision of prey refugia through habitat complexity, might reduce their impacts. However, early detection and complete eradication of individuals located in new regions is advised.