Dempsey as a 16 year old began hopping on trains and travelling west to fight as a professional, and his exact record is not known because sometimes he used the name of Kid Blackie to book himself into fights. This practice continued until 1916. In between, he first appeared as Jack Dempsey in 1914, drawing with Young Herman in six rounds. After that fight, he won six bouts in a row by knockout (as Jack Dempsey), before losing for the first time, on a disqualification in four to Jack Downes. During this early part of his career, Dempsey campaigned in Utah frequently. He followed his loss against Downey with a knockout win and two draws versus Johnny Summerland in Nevada. Three more wins and a draw followed and then he met Downes again, this time resulting in a four round draw.
10 wins in a row followed, a streak during which he beat Summerland and was finally able to avenge his defeat at the hands of Downes, knocking him out in two. Then, three more non-decisions came (early in boxing, there were no judges to score a fight, so if a fight lasted the full distance, it was called a draw or non-decision, depending on the state or country the fight was being held in). In between the non-decisions, Dempsey refused to box with Sam Langford, a Black fighter who is now in the International Boxing Hall Of Fame alongside Jack. Dempsey was always aware that fights with Black opponents could bring negative society reactions.
When the United States entered World War I in 1917, Dempsey worked in a shipyard while continuing to box. After the war, he was accused by some boxing fans of being a draft dodger. It wasn't until 1920 that he was able to clear his name on that account, when evidence was produced showing he had attempted to enlist in the U.S. Army, but was turned down.
Meanwhile, Dempsey went 9-1-4 in 14 bouts in 1917. Among his opponents were Fireman Jim Flynn, the only boxer ever to beat Dempsey by a knockout when Dempsey lost to him in the first round, and Gumboat Smith, a fringe contender stopped by Dempsey.
In 1918, Dempsey boxed 17 times, going 15-1 with one no decision. He avenged his defeat against Flynn by returning the favor, knocking him out in the first round. Among others he beat were Kid Levingsky, a top rated contender of the times.
He began 1919 winning five bouts in a row by knockout in the first round. Then on July 4, he and world Heavyweight champion Jess Willard met at Toledo, Ohio, for the world title. He dropped Willard seven times in round one, and in perhaps one of the biggest bloopers in boxing history, he left the ring after the bell had sounded to finish that round, thinking the referee had stopped the fight and the title was his. Fight footage clearly shows his trainer Jack Kearns claiming for him to get back in the ring. Nevertheless, he went back and finished his work, Willard quitting in his corner at the end of the third round and giving Dempsey the world crown.
In his first defense, he faced friend Billy Miske, knocking him out in three rounds. Years after the fight, it was learned Miske accepted the fight while suffering a terminal disease and needed the money to secure his family after death, which occurred to him two years after challenging Dempsey. Dempsey always expressed regret about that fight and declared he would have given Miske the money he needed if he'd only known of Miske's situation.
One more defense followed, versus Bill Brennan, before he had to face world Light Heavyweight champion Georges Carpentier of France, in what became boxing's first million dollar gate ever. Carpentier had served in the war and was a decorated veteran of the French Army. Ironically, Dempsey's promoter used this angle to promote the fight, since many Americans still regarded Dempsey as a slacker during the war. In a farm that had to be rented to accommodate all the public in New Jersey, Dempsey beat Carpentier by a knockout in four rounds in front of 80,183 fans.
After this fight, Dempsey's fame reached unexpected heights, becoming one of the 1920s top five sports stars, along with baseball's Babe Ruth, tennis' Bill Tilden, American football's Red Grange and golf's Bobby Jones. They were known as the big 5 of sports.
In 1922, he fought just one official fight, a four-round bout against journeyman Jimmy Darcy in Buffalo, New York that was originally scheduled as an exhibition, but was required to be an official fight under New York law. Dempsey also fought several exhibition bouts across the country that year.
In 1923, he had two fights: one against Tommy Gibbons in the small town of Shelby, Montana, a fight which was a financial disaster. Dempsey retained the title by a decision (decisions had already been incorporated by 1923 in boxing), but the town went bankrupt after the fight. In his second match that year, he met Argentina's Luis Firpo in a historic fight at the Madison Square Garden in New York. Firpo became the first hispanic to challenge for the world Heavyweight title, and Dempsey had him down seven times in round one, but Firpo found a combination to the head that dropped Dempsey outside the ring and on all fours before the end of the round. Dempsey hit his head against a writer's typewriting machine, but he recovered, got up at the count of nine and knocked Firpo out in the second round to retain the title.
Dempsey signed a contract to fight Black contender Harry Wills in 1924, but it never occurred. Promoter Tex Rickard was against the match, remembering the riots that occurred after Rickard promoted the James Jeffries versus Jack Johnson bout and fearing a racial repercussion after a bout between Dempsey and a Black opponent.
He got married to actress Estelle Taylor, a Hollywood star, during 1925 and started appearing in some films and doing more exhibition bouts, but he did not defend his title again until 1926. Among those exhibitions, there was a trip to Germany where he and future world champion Max Schmeling boxed a two round exhibition.
Dempsey wasn't quite ready to retire from the ring, and in 1927, he knocked out future world champion Jack Sharkey in the eighth round of an elimination bout for a title shot against Tunney. The rematch took place in Chicago on September 22, 364 days after losing his title to Tunney in their first bout.
Dempsey was losing the fight on points by a wide margin when he knocked Tunney to the canvas with a left and right combination to the chin in the seventh round. By rule, when a fighter knocks down an opponent, he must immediately go to a neutral corner, but Dempsey seemed to have forgotten that rule and refused to immediately move to the neutral corner when instructed by the referee. The referee had to escort Dempsey to the neutral corner, which bought Tunney at least an extra five seconds to recover.
Boxing historians and filmmakers have counted the time Tunney stayed down between 13 and 16 seconds. But, after Dempsey finally went to a neutral corner, the referee started his count, and Tunney got up at the count of nine. Dempsey tried to finish Tunney off before the round ended, but failed to do so. A fully recovered Tunney dropped Dempsey in round eight, easily won the final two rounds of the fight, and retained the title on a unanimous decision. Because of the controversial nature of the fight, it remains known in history as the fight of The Long Count.
He retired after this bout and made countless exhibition bouts. He opened a restaurant in New York City, which he kept open well into the 1960s, and he divorced Taylor and, in July 1933, married Broadway singer Hannah Williams (who herself had just divorced Roger Wolfe Kahn) and had two children by her.
When the United States entered World War II, Dempsey had an opportunity to refute any remaining criticism of his war record of two decades earlier. He volunteered for national service and was commissioned a commander in the U.S. Coast Guard, charged with developing a physical fitness program for U.S. soldiers. Later, he served a morale officer in the Pacific, and in 1945, he became a hero to many when, at age 49, he insisted on going into battle on Okinawa with a group of men he had trained.
Legend says that one time, an elder Dempsey was mugged by a couple of teen thieves, whom he knocked out and held until the police arrived. He made friends with Wills and Tunney after retirement, and had many books written about his life.
He had a record of 62 wins, 6 losses, 8 draws, 5 no decisions and 1 no contest, with 50 knockouts.
He is a member of the International Boxing Hall Of Fame.
Jack Dempsey is buried in the Southampton Cemetery, Southampton, New York.