The replacement of grasslands with monospecific forests of Australian acacia (Acacia melanoxylon) has led to significant changes in habitat structure across the landscapes of Buenos Aires, Argentina, particularly in the Tandilia Mountain System. Given that this invasive tree drastically modifies plant architecture in these mountain grasslands, it is expected to affect the presence and habitat-use patterns of native fauna. We predicted a lower frequency of native carnivorous mammals in areas invaded by exotic acacia compared to native grasslands/scrub environments. To test this prediction, mammal presence was assessed using camera traps (photo-trapping) in the Paititi Private Natural Reserve, located within the Tandilia Mountain System. Throughout four seasonal samplings spanning one year, we evaluated the environmental use and daily activity patterns of two dominant mammalian predators: the Pampas gray fox (Lycalopex gymnocercus) and the Geoffroy's cat (Leopardus geoffroyi). While both species showed preferences towards areas with higher vegetation cover, no preferences were observed regarding the occupation of grassland/scrub environments compared to acacia forests. Regarding activity patterns, the fox was found to be more crepuscular/nocturnal. At the same time, the cat did not exhibit a clear activity pattern, being potentially active at any time of the day. The ecological flexibility may explain why they were not significantly affected by habitat changes in the studied area. Additionally, the remaining patches of native habitat in the region may help mitigate the effects of anthropogenic impact.