Born in Rocky Mount, North Carolina, Leonard left school at the age of 14 to support his family following his father's death. He worked in a hosiery mill and for the Atlantic Coast Line Railroad. He later earned a GED by correspondence. He began playing semiprofessional baseball while working for the railroad, then decided to pursue his living with the sport.
He began his Negro league career in 1933 with the Brooklyn Royal Giants, then moved to the legendary Homestead Grays in 1934, the team he played for until his retirement in 1950. The Grays of the late 1930s through the mid-1940s are considered one of the greatest teams of any race ever assembled. Leonard batted fourth in their lineup behind Josh Gibson. Since Gibson was known as the "Black Babe Ruth" and Leonard was a first baseman, Buck Leonard was inevitably called the "Black Lou Gehrig." Together, the pair was colloquially known as the "Thunder Twins" or "Dynamite Twins". From 1937 to 1945 the Grays won 9 consecutive Negro National League championships. Leonard led the Negro leagues in batting average in 1948 with a mark of .395, and usually either led the league in home runs or finished second in homers to teammate Gibson.
In 1952, Leonard was offered a major league contract, but he believed that at age 45 he was too old and might embarrass himself and hurt the cause of integration. He may well have underestimated his own longevity, however, since he batted .333 in 10 games in the Class B Piedmont League the following year, and played in Mexico through 1955, where the level of play was very high.
Leonard suffered a stroke in the 1980s. In 1994, the Major League Baseball All-Star Game was held in Pittsburgh, hometown of the Grays, and the 88-year-old Leonard was named an honorary captain. He appeared wearing a model of a Grays uniform. Shortly before his death in 1997, Leonard was the subject of a North Carolina General Assembly proclamation recognizing his contributions to baseball. His death late that year stemmed from complications of his earlier stroke.
In 1999, he ranked Number 47 on The Sporting News' list of the 100 Greatest Baseball Players, one of five players so honored who played all or most of their careers in the Negro leagues, and was nominated as a finalist for the Major League Baseball All-Century Team.
The first official statistics for the Negro leagues were compiled as part of a statistical study sponsored by the National Baseball Hall of Fame and supervised by Larry Lester and Dick Clark; a research team collected statistics from thousands of boxscores of league-sanctioned games. The first results from this study were the statistics for Negro league Hall of Famers elected prior to 2006, which were published in Shades of Glory by Lawrence D. Hogan. These statistics include the official Negro league statistics for Buck Leonard.