Plants are shifting their ranges towards higher elevations in response to global warming, yet such shifts are occurring at a rate slower than is needed to keep pace with a rapidly changing climate. There is, however, an almost complete lack of knowledge on seed dispersal across altitude, a key process to understand what constrains climate-driven range shifts. Here, we report the first direct empirical evidence on altitudinal seed dispersal mediated by two common frugivorous mammals: the red fox Vulpes vulpes and the pine marten Martes martes. We conducted a three-year (bait-marking) experiment in a mountainous region of Spain. We offered experimental fruits containing colour-coded seed mimics at feeding stations that simulated source trees. The colour codes allowed us to identify the exact origin of seed mimics found later in mammal scats. Nearly half (47%) of the dispersal events occurred towards higher elevations, despite only ca 25% of the study area being above the average altitude of the feeding stations (1344 m). Seeds dispersed uphill gained an average of 106 m (median = 111 m) and a maximum of 288 m, greatly exceeding the estimated requirements to escape warming (35.4 m per decade). Yet, foxes mediated much more uphill seed dispersal than martens (57% and 26% of dispersal events, respectively), which can be explained by between-disperser differences in home range size and habitat specificity. Dispersers with larger home ranges move farther and potentially disperse more seeds to higher altitudes, while habitat generalism is necessary to transport seeds above vegetation belts delimiting contrasting habitat types. We discuss how both traits (home range size and habitat specificity) can be used to infer altitudinal seed dispersal across disperser species and mountainous landscapes.