Fitzsimmons was born in Helson, Cornwall, England and came to New Zealand at the age of nine along with his parents, brothers and sisters. His family settled in Timaru and Bob became a blacksmith in his brother Jarrett's blacksmith shop.
Between 1880 and 1881, Fitzsimmons was the champion of the Jem Mace tournament in New Zealand. It is in New Zealand that some say he officially began his career as a professional boxer later in 1881. In a bout that is not clear whether he was paid for or not, he knocked out Herbert Slade in two rounds.
He had six fights there, two of them bare-knuckles events. He won five and lost 1 but it's unclear whether any of those bouts were for money or not.
Boxing record books show that he officially began boxing professionally in 1883, in Australia. He beat Jim Crawford by a knockout in three there. Fitzsimmons had his first 28 fights in Australia, where he lost for the Australian Middleweight title in a bout that was rumored to have been fixed, and where he also won a fight by knockout while in the floor: When Edward Starlight Robins dropped Fitzsimmons to the canvas in round nine of their fight, he also broke his hand and could not continue, therefore, Fitzsimmons was declared winner by knockout.
Then, on January 14 of 1891, in New Orleans, he received his first world title try, against Jack (Nonpareil) Dempsey. He knocked out Dempsey (from whom the latter Jack Dempsey would take his name) in the 13th round to become the world's Middleweight champion. On July 22, his fight with Jim Hall was broken off by police.
Fitzsimmons spent the next two years fighting non-title bouts and exhibitions until giving Hall a chance at the title in 1893. He retained the crown by a knockout in round four. He spent the rest of that year doing exhibitions, and on June 2, he was supposed to have a two way exhibition where he would demonstrate in public how to hit the boxing bag and then how to box against a real opponent. Reportedly, two freak accidents happened that day, when Fitzsimmons hit the bag so hard it broke, and then his opponent of that day allegedly slipped, getting hit in the head and the boxing exhibition cancelled.
After vacating the Middleweight crown, Fitzsimmons began campaigning among Heavyweights, who usually outweighted him by large amounts of weights. One of his fights, against Tom Sharkey, was refereed by none other than Wyatt Earp, the famous cowboy. He was declared loser on a disqualification because he hit Sharkey while Sharkey was down.
Nevertheless, Fitzsimmons was able to challenge for the world's Heavyweight title in 1897. On March 17 of that year, he became world Heavyweight champion, knocking out Jim Corbett in round 14. He spent the rest of the year doing exhibitions, and in 1898, his only activity of note was a bout that was supposed to happen against former world champion John L. Sullivan. That fight never happened, however, because boxing was outlawed in New York at the time, and that would have been the fight's host city, but the police prevented it from happening.
In 1899, Fitzsimmons and James J. Jeffries were able to box in New York without the police intervening, probably at an under-ground club. Jeffries lifted he world Heavyweight crown from Fitzsimmons with an 11 round knockout.
In 1902, he and Jeffries had a rematch, once again with the world Heavyweight crown at stage. Attempting to become the first boxer ever to regain the world's Heavyweight crown, he lost to Jeffries by a knockout in round eight.
September of 1903 was a tragic month for Fitzsimmons as his rival, Con Coughlin, died the day after suffering a one round knockout at the hands of Fitzsimmons. But less than two months later, Fitzsimmons made history by defeating world Light-Heavyweight champion George Gardner by a decision in 20 rounds to become the first member of the exclusive club of boxing world champions in three or more divisions.
Soon after, he went back to the Heavyweights, where he kept fighting until 1914, with mixed results. He boxed Jack Johnson, and his fight with Bob KO Sweeney is believed to be the first boxing fight in history to be captured on film.
It should be clarified that, despite the fact that Fitzsimmons was a world champion in the Middleweight, Light Heavyweight and Heavyweight divisions, he is not considered by historians to be the first world Light Heavyweight champion to become world Heavyweight champion. This, because of the fact that he won the Heavyweight title before winning the Light Heavyweight belt. Michael Spinks is considered to be the first Light Heavyweight world champion to win the Heavyweight belt as well. Roy Jones Jr recently joined Fitzsimmons and Spinks in being the only men to have won world championships at both Light Heavyweight and Heavyweight.
His exact record is unknown, as boxing records were barely well kept during his era, but Fitzsimmons was quoted as saying he had more than 350 fights, which could have been an exageration from his part.
He is a member of the International Boxing Hall of Fame.