Braddock pursued boxing, turning pro at the age of 21, fighting as a light heavyweight. His first fight in a ring occurred on November 27, 1923. After three years, Braddock's record was 44–2–2 (.938), with 21 knockouts.
In 1928, Braddock pulled off a major upset by knocking out highly regarded Tuffy Griffiths. The following year he earned a chance to fight for the title, but he narrowly lost to Tommy Loughran in a 15-round decision. Braddock was greatly depressed by the loss and badly fractured his right hand in several places in the process.
His next 33 fights were significantly less successful, with a 11–20–2 (.364) record. With his family in poverty during the Great Depression, Braddock was forced to give up boxing and work as a longshoreman. Due to frequent injuries to his right hand, Braddock compensated by using his left hand during his longshoreman work, and it gradually became stronger than his right. He always remembered the humiliation of having to accept government relief money, but was inspired by the Catholic Worker Movement, a Christian social justice organization founded by Dorothy Day and Peter Maurin in 1933 to help the homeless and hungry. After his boxing comeback, Braddock returned the welfare money he had received and made frequent donations to various Catholic Worker Houses, including feeding homeless guests with his family.
Max Baer fight
In 1934, Braddock was given a fight with the highly touted John "Corn" Griffin. Although Braddock was intended simply as a stepping stone in Griffin's career, he knocked out the "Ozark Cyclone" in the third round. Braddock then fought John Henry Lewis, a future light heavyweight champion. He won in one of the most important fights of his career. After defeating another highly regarded heavyweight contender, Art Lasky, whose nose he broke during the bout on March 22, 1935, Braddock was given a title fight against the World Heavyweight Champion, Max Baer.
Considered little more than a journeyman fighter, Braddock was hand-picked by Baer's handlers because he was seen as an easy payday for the champion, despite his recent impressive victories. Instead, on June 13, 1935, at Madison Square Garden Bowl, Braddock won the Heavyweight Championship of the World as the 10-to-1 underdog in what was called "the greatest fistic upset since the defeat of John L. Sullivan by Jim Corbett".
During the fight, a dogged Braddock took a few heavy hits from the powerful younger champion (30 years versus 26 years for Baer), but Braddock kept coming, wearing down Baer, who seemed perplexed by Braddock's ability to take a punch. In the end, the judges gave Braddock the title with a unanimous decision.
Braddock suffered from problems with his arthritic hands after injuries throughout his career and, in 1936, his title defense in Madison Square Garden against the German Max Schmeling was canceled under suspicious circumstances. Braddock argued he would have received only a US$25,000 purse against Schmeling, compared to $250,000 against rising star Joe Louis. There was also concern that if Schmeling won, the Nazi government would deny American fighters opportunities to fight for the title. Finally, American commentators had expressed opposition to the fight in light of the connections between Schmeling and Adolf Hitler, with whom the German fighter had been associated after his earlier victory over Louis.