The first known American to be paid for playing baseball is Jim Creighton, who started playing baseball in the 1850s. Creighton was renowned for his skill as a pitcher and batter, making him a highly desirable player, and in the 1860s, the Excelsiors of Brooklyn paid him to play for their team.
At this point in history, baseball teams were somewhat organized as clubs that played for audiences, traveled and wore uniforms. However, payment wasn't part of the game, and the Excelsior club paid Creighton subtly rather than overtly offering him a salary. Another player of this era named George Flanley may also have received money.