Proving his status as a contender, Lewis fought accomplished boxer Tommy Lowe on January 2, 1905, in a fifteen round draw in Philadelphia.

On May 19, 1905, in only his second year of boxing, he met Benny Yanger, an exceptional featherweight, whom he defeated in a first round TKO at the armory in Baltimore.

Partly accounting for his legendary reputation, three years earlier in 1902, Yanger had been one of the few boxers to ever defeat Abe Attell during his early reign as featherweight champion.

[3][9] Moving up from featherweight in 1905, Lewis fought leading lightweight contender Young Erne six times, losing only twice, though drawing the remainder of his fights.

He fought Unk Russell twice in October and November 1905, winning decisively in two six round bouts in Philadelphia, and further demonstrating his potential as a serious contender.

On June 15, 1906, Lewis took on the great reigning black Lightweight World Champion Joe Gans in Philadelphia at the National Athletic Club.

Though the fight was a six round no decision, Lewis was viewed as a far more dangerous competitor having compared favorably with one of history's greatest lightweights.

On January 25 of that year, he fought Willie Lewis for three rounds in New York, losing in the newspaper decision of the Boston Globe.

[9][10] Having stood the test with the great lightweight Gans, Lewis moved up to welterweight, and defeated Rube Smith convincingly on January 22, 1907, in an eight-round knockout in Denver.

Though highly unpopular with the crowd, the decision against Sullivan by referee Reddy Gallagher may have been influenced by Lewis's style, speed, and defensive skills.

A year later, when Lewis met world welterweight champion Honey Mellody, he added both weight and strength to his natural skills of speed and technique.

[11] On May 6, 1907, Lewis fought accomplished light welterweight Jimmy Gardner in a six round bout, further demonstrating his status as a contender.

On January 23, 1908, he defeated the top world championship welterweight claimant Frank Mantell at Edgewood Athletic Club in New Haven, Connecticut in a third round TKO, giving real substance to his claim to the title.

At the zenith of his career, Lewis secured the Welterweight Championship of the World defeating William "Honey" Mellody, the most recent holder of the title, on April 20, 1908, at the armory in Boston in a fourth round TKO, although no single sanctioning organization was recognized worldwide at the time.

[3] On December 14, 1908, Harry had a close bout with one of his frequent opponents, welterweight contender Willie Lewis at the New Haven, Connecticut, Grand Opera House.

Among the two preliminary bouts, the first was a close fight between New London native Abe Hollandersky and Maurice Lemoine of Webster, Massachusetts.

BoxRec records this bout as the loss of the Middleweight World Championship which Lewis claimed to have taken on February 22, 1911, from Blink McClosky previously at the Hippodrome.

The Middleweight World Championship had been vacated by Stanley Ketchell the previous year, and remained vacant all of 1911, though no sanctioning bodies active today recognize Lewis's claim to the title.

On December 13, 1911, he lost to French boxing great Georges Carpentier at the Cirque de Paris in front of large enthusiastic crowds.