On February 25, 1901, Johnson fought Joe Choynski in Galveston. Choynski, a popular and experienced heavyweight, knocked out Johnson in the third round. Prizefighting was illegal in Texas at the time and they were both arrested. Bail was set at $5,000 which neither could afford. The sheriff permitted both fighters to go home at night so long as they agreed to spar in the jail cell. Large crowds gathered to watch the sessions. After 23 days in jail, their bail was reduced to an affordable level and a grand jury refused to indict either man. However, Johnson later stated that he learned his boxing skills during that jail time. The two would remain friends.
Johnson attested that his success in boxing came from the coaching he received from Choynski. The aging Choynski saw natural talent and determination in Johnson and taught him the nuances of defense, stating "A man who can move like you should never have to take a punch".
Throughout his career Johnson built a unique fighting style of his own, which was not customary to boxing during this time. Though Jack would typically strike first, he would fight defensively, waiting for his opponents to tire out, while becoming more aggressive as the rounds went on. He often fought to punish his opponents through the rounds rather than knocking them out, and would continuously dodge their punches. He would then quickly strike back with a blow of his own. Jack often made his fights look effortless, and as if he had much more to offer, but when pushed he could also display some powerful moves and punches. There are films of his fights in which he can be seen holding up his opponent, who otherwise might have fallen, until he recovered.
His style of playing with his opponents was very effective, but was criticized by the press as being a cowardly fighting approach. In contrast, world heavyweight champion "Gentleman" Jim Corbett, who was white, had used many of the same techniques a decade earlier, and was praised by the press as "the cleverest man in boxing.
Johnson beat former black heavyweight champ Frank Childs on October 21, 1902. Childs had twice won the black heavyweight title and continued to claim himself the true black champ despite having lost his title in a bout with George Byers and then, after retaking the title from Byers, losing it again to Denver Ed Martin. He still made pretence to being the black champ and claimed the unrecognized black heavyweight title as well. Johnson won by a TKO in the 12th round of the scheduled 20 rounder, when Childs's seconds signaled he couldn't go on. (He claimed he had dislocated his elbow.) The defeat by Johnson forever ended Childs's pretensions to the black heavyweight crown.