Riding bicycles off-road goes back to the beginning of cycling itself. Road racing cyclists have long used cyclo-cross as a means of keeping fit during the winter, eventually becoming a sport in its own right with the first world championship in 1950. The French Velo Cross Club Parisien (VCCP) comprised about twenty-one young cyclists from the outskirts of Paris, who between 1951 and 1956 developed a sport that was remarkably akin to present-day mountain biking. The Roughstuff Fellowship was established in 1955 by off-road cyclists in the UK.
However the mountain bike has its origins in the modified heavy cruiser bicycles used for freewheeling down mountain trails in Marin County, California, U.S.A. in the mid-late 1970s. At the time, there was no such thing as a mountain bike. The earliest ancestors of modern mountain bikes were based around frames from cruiser bicycles such as those made by Schwinn. The Schwinn Excelsior was the frame of choice due to its geometry. Riders used balloon tired cruisers and later modified them with gears and motocross style handlebars. They were called Klunkers. The term would also be used as a verb since the term "mountain biking" was not yet in use. Riders would race down mountain fireroads, causing the hub brake to burn the grease inside, requiring the riders to repack the bearings. These were called "Repack Races" and triggered the first innovations in mountain bike technology as well as the initial interest of the public. The sport originated in the U.S. state of California, on Marin county's famous mountain, Mount Tamalpais.It was not until the late 1970s and early 1980s that road bicycle companies started to manufacture mountain bicycles using high-tech lightweight materials. Joe Breeze is normally credited with introducing the first purpose-built mountain bike in 1978. Tom Ritchey then went on to make frames for a company called MountainBikes which was a partnership between Gary Fisher, Charlie Kelley and Tom Ritchey. Tom Ritchey, with his skills in frame building, also built the original bikes. The company's 3 partners ran into rough times and finally fell apart at the 1983 trade show. The designs were basically road bicycle frames (with heavier tubing and different geometry) with a wider frame and fork to allow for a wider tire. The handlebars were also different in that they were a straight, transverse-mounted handlebar, rather than the dropped, curved handlebars that are typically installed on road racing bicycles. Also, some of the parts on early production mountain bicycles were taken from the BMX bicycle. The first mass-produced mountain bikes were produced by Specialized in 1983 and were copies of Tom Ritchey's frames, but they were not fillet-brazed, and were made in Japan. They were configured with 15 gears.

Mountain bikes have wide, knobby tires for extra traction and shock absorption. In recent years, front wheel suspension has become the norm and full front and rear suspension is becoming increasingly common. Some mountain bikes are also fitted with bar ends on the handlebars, but with the increase in popularity of riser handlebars (as opposed to a flat straight handlebar) fewer riders use bar end extensions. The bikes normally have 26 in (559 mm) wheels, but since 2002 some models have been available with 29 in (622 mm) wheels, which is the same diameter most commonly used for road bikes (also known as 700c). Some dirt jumping bikes also have 24in wheels.