Managing urban-adjacent lands for both wildlife conservation and recreational opportunities can be a delicate balance. Understanding the impacts of activities growing in popularity, such as mountain biking, can be especially challenging. This study used data from a social fitness app, Strava Metro—which provided an index of recreational use--to quantify and separate the effects of hiking and mountain biking on wildlife in parks in Marin County, California, USA. Combining this user data with mammal frequency data from a community-science camera trap project, we assessed how mammals spatially and temporally responded to distance from trails with respect to levels of hiking and mountain biking activity. Of the five primarily non‐nocturnal mammals in our study area, four were either spatially or temporally less frequent near trails versus away from trails. Mule deer (Odocoileus hemionus) were moderately sensitive to sites with high levels of mountain bike use. When accounting for both hiking and biking use, both mule deer and brush rabbits (Sylvilagus bachmani) were less frequent at high‐use mountain bike sites. Strava Metro provided useful data for these analyses, showing potential as a resource for managing mountain biking effects on public lands as mountain and e‐bike prevalence increases.