He was born in Galveston, Texas and is reputed to have fought his first fight, a sixteen round victory, aged fifteen. He turned professional around 1897, fighting in private clubs. He was briefly arrested in 1900 as boxing was illegal in Texas.
He won his first title on February 3, 1903, beating 'Denver' Ed Martin over twenty rounds for the Colored Heavyweight Championship. His efforts to win the full title were thwarted as white champions refused to face him. He fought the former champion Bob Fitzsimmons in July 1907 and knocked him out in two rounds.
He eventually won the World Heavyweight Title on December 26, 1908 when he fought the World Heavyweight Champion, Canadian Tommy Burns in Sydney, Australia, after following him there. The fight lasted fourteen rounds before being stopped by the police. The title was awarded to Johnson on a referee's decision as a T.K.O.
As title holder he had to face a series of fights with "great white hopes", often as exhibition matches. In 1909 he beat Victor McLaglen, Frank Moran, Jack O'Brien, Tony Ross, Al Kaufman, and the middleweight champion Stanley Ketchel. But on July 4, 1910 he defeated James J. Jeffries, a champion who had earlier turned him down, with a K.O. in the fifteenth round in front of 22,000 people, earning Johnson $115,000. His victory sparked race riots and certain states banned the filming of Johnson's victories over white fighters.
In January 1911 he married Etta Duryea. But in September Etta committed suicide. Johnson, undefeatable in the ring, came under increasing external pressure - in June 1912 he was indicted for smuggling a diamond necklace and in October he was arrested under a section of the Mann Act, initially enacted to combat prostitution but never before used in prosecution, for transporting his fiancÍ, a white woman named Lucille Cameron, across state lines. Despite the looming trial he married Cameron in December 1912. He went to trial in Chicago in 1913 and on May 14 he was convicted and sentenced to a year and a day plus a fine of $1000.
In mid-June he fled the United States while free pending appeal. He continued fighting, mainly in exhibition bouts, but on April 5, 1915 the 37 year old lost his title to Jess Willard in Havana. With a crowd of 25,000 for the scheduled 45 round fight Johnson was K.O.'d in the 26th round. He fought a number of bouts in Mexico before returning to the US on July 20, 1920 and surrendering to Federal agents. He was sent to Leavenworth to serve his sentence, released on July 9, 1921.
He continued fighting, but age was catching up with him. After two losses in 1928 he participated only in exhibition bouts. He opened a night club in Harlem. He divorced Lucille Cameron in 1924 and married Irene Pineau.
He died in a car crash near Raleigh, North Carolina in 1946 and was buried in the Graceland Cemetery, in Chicago, Illinois. He was admitted to the Boxing Hall of Fame in 1954
His fighting style was very distinctive, he always began a bout cautiously before slowly building up over the rounds into a more aggressive (and abusive) fighter. He often fought to punish his opponent rather than knock them out, endlessly avoiding their blows and striking with swift counters. He always gave the impression of having much more to offer and if pushed he could really damage an opponent.
In person he was reputedly arrogant, flamboyant and reckless. Traits that did little to endear him to many people. His marrying of White women also enraged many members of the White race, and it has always been rumored in boxing circles that he threw away the fight with Willard after being promised a deal by American lawmen where he could return to visit his ill mother without getting arrested.