He was known for his enormous strength and stamina. Using a technique taught to him by his trainer, former Welterweight and Middleweight Champion Tommy Ryan, Jeffries fought out of a crouch with his left arm extended forward. He was able to absorb tremendous punishment while wearing his opponents down. A natural left-hander, he possessed one-punch knockout power in his left hook, and brawled his way to the top of the rankings. He is perhaps most famous for being America's "Great White Hope", since the nation expected him to come out of his retirement to beat the African-American boxer Jack Johnson, who was at the time the Heavyweight Champion.
Jeffries stood 6 ft 11⁄2 in (1.87 m) tall and weighed 225 pounds (102 kg) in his prime. He could run 100 yards (91 m) in just over ten seconds, and could high jump over 6 feet (180 cm).
In 1891, Jeffries' father moved his family from their Ohio farm to Los Angeles, California. James worked for a while as a boilermaker before going into boxing. In later life, "The Boilermaker" was one of his professional nicknames.