Biological invasions by alien species have increased worldwide as a consequence of globalisation and the growing connectivity among regions. In Italy, alien mammalian species account for more than 10% of the country's mammal fauna, highlighting their ecological relevance in the Mediterranean region. To assess the environmental impact of these species, we conducted a comprehensive review of the scientific literature documenting their effects on native biodiversity in Italy. All impact records were systematically assessed using the EICAT framework, allowing a standardised classification of both impact mechanism and magnitude. Competition, Grazing/herbivory/browsing, and Predation emerged as the most frequently reported impact mechanisms, with most records assigned to the lower impact categories (Minimal Concern and Minor). Nevertheless, some alien mammals, notably Procyon lotor and Sciurus carolinensis, were reported to exert severe impacts on native taxa at the population level, posing significant challenges for biodiversity conservation and invasive species management. Despite the limited availability of impact data for several species, our study provides the first comprehensive synthesis of the documented environmental impacts of alien mammals in Italy. Integrating national evidence with information from European and global assessments can improve risk assessment and help identify potential impacts before they are documented in Italy. For poorly studied or localised species, adopting a precautionary approach may prevent delays in management actions and reduce future impacts on native biodiversity.