[2][3] Mullin began playing professional baseball in 1901 with the Fort Wayne Railroaders of the Western Association.

The owner of the Fort Wayne club, Isadore Mautner, sought to have Mullin charged criminally with taking money under false pretenses and perjury.

[8] In July, Mullin was removed by police when the Tigers' train stopped in Fort Wayne on returning from St. Louis.

A judge promptly released Mullin, holding that Mautner's remedy was a civil action for damages.

[9] Mautner later persuaded a grand jury to bring criminal charges against Mullin,[10] and the saga continued to capture headlines well into 1903.

[13][14] In his first season in Detroit, Mullin immediately showed his ability as a workhorse and as a batsman, but struggled with his pitch control and overall effectiveness.

At the plate, he compiled an impressive .325 batting average and .367 on-base percentage with four doubles, three triples, and 11 RBIs.

He ranked among the American League's leading pitchers with six shutouts (second), 41 games (second), 170 strikeouts (fifth), 320+2⁄3 innings pitched (fifth), and a 2.25 ERA (sixth).

[2] On September 19, 1903, Mullin hit his first major league home run, described by the Detroit Free Press as a "mighty clout" that "lifted the ball over center field fence into Twenty-ninth street.

"[19] In 1904, Mullin had another strong year, pitching a club record 42 complete games with a 2.40 ERA (below the league average of 2.60).

[22] And control issues persisted as he led the league in walks allowed for the fourth consecutive years.

[2] In 1907, the Detroit club hired Hughie Jennings as its manager, and with the bats of Ty Cobb and Sam Crawford, the team finally had a winning combination, as they won three straight American League pennants from 1907 to 1909.

During the Tigers' pennant years, Mullin remained the pitching staff's workhorse, appearing in 125 games and compiling a 66–41 record with a 2.63 ERA.

[3] In November 1909, a group of Detroit Tigers players toured Cuba and played 12 exhibition games against two integrated Cuban teams, Habana and Almendares.

Demonstrating the high level of play in Cuba, the Tigers lost 8 of the 12 games to the integrated Cuban baseball teams.

On April 20, 1912, with a crowd of 26,000 in attendance, Mullin was given the honor of pitching the first game at the new Navin Field, beating the Indians 6–5 in 11 innings.

[26] On May 21, 1912, Mullin shut out the Washington Senators, earning his 200th career win — a 2–0 pitching duel with Walter Johnson.

[29] In Washington, Mullin joined a pitching staff led by Walter Johnson who won 36 games in 1913.

[30] On January 1, 1914, Mullin announced that he had come to terms to play for the Indianapolis Hoosiers of the newly formed Federal League.

Mullin was a powerfully built pitcher (often referred to as "burly") with an intimidating fastball, perhaps even more so due to his imperfect control.

His career record of 2.56 assists per game also ranks seventh in major league history for pitchers.

[38] Longtime Detroit sports writer H. G. Salsinger wrote that, while Mullin had an excellent fastball and a good curveball, "most of his success was due to smartness rather than stuff.

"[39] According to Salsinger, Mullin would try to distract batters with antics in between pitches – combing the pitcher's box, adjusting his belt, cleaning non-existent dirt from his glove, tying or removing his shoes –all in the hope of upsetting the batter and inducing him to swing at a bad pitch.

According to the next day's account in the Detroit Free Press, Mullin "hit center field with a triple.

"[41] Yet, as one author noted: "The pitching prowess and significant achievements of George Mullin seem to have faded away on the brittle pages of baseball history.