In 1901, William S. Harley, age 21, drew up plans for a small engine with a displacement of 7.07 cubic inches (116 cc) and four-inch (102 mm) flywheels. It was finished in 1903 and Upon completion the boys found their power-cycle unable to conquer Milwaukee's modest hills without pedal assistance. Will Harley and the Davidsons quickly wrote off their first motor-bicycle as a valuable learning experiment.

By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world. Their motorcycles were sold by dealers in 67 countries. Production was 28,189 machines. In 1921, a Harley-Davidson, ridden by Otto Walker, was the first motorcycle ever to win a race at an average speed of over 100 mph (160 km/h)

Hollywood managed to damage Harley's image with many outlaw biker gang films produced from the 1950s through the 1970s, following the Hollister riot on July 4, 1947. "Harley-Davidson" for a long time was synonymous with the Hells Angels and other outlaw motorcyclists.

The engine design is unique and causes the pistons to fire at uneven intervals. This is due to an engineering tradeoff to create a large, high-torque engine in a small space. This design choice has been sustained because of the strong connection between the distinctive sound and the Harley-Davidson brand. This design, which is covered under several United States patents, and gives the Harley-Davidson V-twin its unique choppy "potato-potato" sound.

Beginning in 1920, a team of farm boys consistently won races. The group had a hog, or pig as their mascot. Following a win, they would put the pig (a real one) on their Harley and take a victory lap. In 1983, the Motor Company formed a club for owners of its product taking advantage of the long-standing nickname by turning "hog" into the acronym HOG., for Harley Owners Group.

On August 15, 2006, Harley-Davidson Inc. had its NYSE ticker symbol changed from HDI to HOG.