The color line, also known as the color barrier, in American baseball excluded players of black African descent from Major League Baseball and its affiliated Minor Leagues until 1947. Racial segregation in professional baseball was sometimes called a gentlemen's agreement, meaning a tacit understanding, as there was no written policy at the highest level of organized baseball, the major leagues. A high minor league's vote in 1887 against allowing new contracts with black players within its league sent a powerful signal that eventually led to the disappearance of blacks from the sport's other minor leagues later that century, including the low minors. After the line was in virtually full effect in the early 20th century, many black baseball clubs were established, especially during the 1920s to 1940s when there were several Negro leagues. During this period, American Indians and native Hawaiians, e.g. Prince Oana, were able to play in the Major Leagues. The color line was broken for good when Jackie Robinson signed with the Brooklyn Dodgers organization for the 1946 season. In 1947, both Robinson in the National League and Larry Doby with the American League's Cleveland Indians appeared in games for their teams.

Major League Baseball (MLB) is a professional baseball league and the highest level of organized baseball in the United States and Canada. One of the big four major leagues, MLB comprises 30 teams, divided equally between the National League (NL) and the American League (AL), with 29 in the United States and 1 in Canada. Formed in 1876 and 1901, respectively, the NL and AL cemented their cooperation with the National Agreement in 1903, making MLB the oldest major professional sports league in the world. They remained legally separate entities until 2000, when they merged into a single organization led by the Commissioner of Baseball. MLB is headquartered in Midtown Manhattan.