When the American League was created, Fultz joined the Philadelphia Athletics of Connie Mack in 1901, appearing at shortstop and second base, and later moved to center field. Fultz led his team with 36 stolen bases and hit .292 with 95 runs scored. His most productive season came in 1902, when he stole 44 bases, hit a career-high .302, and led the American League with 109 runs. On September 4, he stole second base, third and home, in the second inning of a game against the Detroit Tigers.
From 1903 to 1905, Fultz played for the New York Highlanders when Clark Griffith managed the team. During the offseason and in his spare time he attended New York Law School, passing the New York bar exam. Fultz averaged 30 stolen bases for season with a high 42 in 1905. That season, he suffered a late September collision with teammate Kid Elberfeld, breaking his nose and jaw, and retired at 31 age.
Fultz got his first coaching job in 1898 as the eighth head football coach at the University of Missouri. Missouri finished with a 1–4–1 record under Fultz. He returned to coach the Tigers for one game in 1900, a victory, before handing over the reins to his former Brown teammate, Fred W. Murphy. Ernest Cleveland "E.C." White coached Missouri to a 9–2 record in 1899.
In 1906, Fultz became a practicing attorney. In 1912, he created a furor in baseball by unionizing major league players in an organization called the Players Fraternity. Ty Cobb and Christy Mathewson were among its officers. The group threatened to strike in 1917, but the walkout was averted after Fultz obtained some concessions for the players. The union was disintegrated during World War I.