When did mountain biking start?
Mountain biking as a sport came into existence in various areas in the USA in the late 1970s and early 1980s.  While many groups of riders in different US locations claim the birthright to mountain biking, it's clear that there wasn't a single person nor a single location that served as the starting point for the sport.  Riders in Crested Butte, CO, Marin County, CA, Mill Valley, CA, Cupertino, CA, and several other locations contributed significantly to the birth of the sport.

It wasn't until the late 1970s and early 1980s that bicycle companies began manufacturing mountain bikes with specific high-tech lightweight materials that popularized the sport.  Joe Breeze (Marin County) is often credited with building the first, purpose-built mountain bike in 1978.

It's important to note that there were mountain bike concepts and prototypes built well before this period, with the earliest concept belonging to the off-road bicycles used by the Buffalo Soldiers in 1896.