Sigfried Bettmann of Coventry, England -- the company name was Triumph Motorcycles..the year: 1914
No one person invented the dirt bike. People began to modify their bikes to ride in the dirt, starting with knobby tires and better suspensions, changing gearing/sprockets, frames, doing everything possible to lighten and strengthen them, getting rid of parts not needed in dirt riding (lites, horn, etc.).
The Motocross of Nations race was created in 1947