National League (NL, baseball) 1876-2016
American League (AL, baseball) 1901-2016
National Football League (NFL) 1920-2016
American Football Conference (AAFC) 1946-1949
National Basketball Association (NBA) 1946-2016
Prominent "first" American born black players:
1920: Fritz Pollard Akron Pros, Bobby Marshall Rock Island Independents; Pollard was also the first black head coach in the NFL
1946 Kenny Washington, Woody Strode roommates Los Angeles Rams
AAFC: Sept. 6, 1946 Marion Motley, Bill Willis roommates Cleveland Browns; both played with the Browns through 1953; Browns joined the NFL in 1950.
NL: April 15, 1947 Jackie Robinson, Aug. 26, 1947 Dan Bankhead (pitcher; home run first at bat) Brooklyn Dodgers
AL: July 5, 1947 Larry Doby Cleveland Indians
NBA: Oct. 31, 1950 Earl Lloyd Rochester Royals, Nov. 1, 1950 Chuck Cooper Boston Celtics, Nov. 3, 1950 Nat "Sweetwater" Clifton New York Knicks
Unlike the two pairs of football players who preceded him by one year, Jackie Robinson was alone on the Dodgers for four months until Dan Bankhead joined the team. Hall of Fame catcher Roy Campanella made his debut April 20, 1948. Why Robinson was alone is a mystery.
In 1947 baseball was the dominant team sport in the United States in a way that we cannot imagine now. Robinson's college football teammates at UCLA in 1939, Kenny Washington and Woody Strode, may have been a big story on the Rams in Los Angeles but it was not much of a factor in the dynamics that Robinson encountered. In other words, they probably did not make it easier for Robinson as he probably did for Campanella.
The significance of Robinson should not be diminished because there were black players before 1901, including catcher Fleet Walker, who played in the American Association (AA) in 1884.
If we consider the modern era of professional baseball as starting in 1901, then the major leagues were racially segregated for 46 years and have been integrated for 70 years. Jackie Robinson lived January 31, 1919 to Oct. 24, 1972. He was 53 when he died of a heart attack.