Mert Lawwill was born in Boise, Idaho. He started his racing career as an amateur racer on the local TT track in Boise and, later, scramble races across the United States Northwest. In 1961, he moved to Los Angeles, California so that he could race at the Ascot Park race track which, at the time was the epicenter of dirt track racing. He gained sponsorship from Dudley Perkins, a Harley-Davidson dealer in San Francisco. It was during this time that Lawwill began to learn about modifying motorcycle frames for racing competitions. By 1963, he had become a professional rider and in 1964 he signed a contract to compete for the Harley-Davidson factory racing team with whom he would remain for the rest of his racing career.
Lawwill won his first AMA national race at the Sacramento Mile on September 19, 1965. In 1969, Lawwill won the AMA Grand National Championship and, was voted AMA's Most Popular Rider of the Year. His defense of his Grand National Championship during the 1970 season became the subject of Bruce Brown's 1971 motorcycle documentary film, On Any Sunday co-starring actor Steve McQueen and off-road racer Malcolm Smith. He continued to compete for the AMA Grand National Championship until 1977 when, he retired at the age of 37 due to an inner-ear disorder that affected his balance. He accumulated 161 career AMA Grand National finishes and won 15 Grand National races during his 15-year racing career.