Brian Helgeland does not cite a specific source for the script of 42. Rather it seems that he pulls from a few different pieces of past work and incorporates his own creativity into the end product that viewers see in theaters. The most prominent and definitive resource he used was Robinson’s autobiography I Never Had it Made (1972). Many of the same events that are outlined in his biography are portrayed in the film, with roughly an equivalent amount of time dedicated in the film as Jackie did in the book: situations such as Jackie’s relationship with Rachel, his first meeting with Branch Rickey, the National Anthem before the beginning of the 1947 World Series, racial bombardment by the Phillies dugout and manager Ben Chapman, and his embrace with Pee Wee Reese prior to a game in Cincinnati. Each piece was clearly important to Jackie because they all made it into his biography, and Helgeland respects this fact by including them in the film. The main difference, of course, is the length of time 42 covers. The choice Helgeland makes to only look at the years from 1945–1947 -- basically only Jackie’s initial baseball career and the breaking of the color barrier -- is different from the biography in which Robinson touches on a much broader range of topics both before and after his playing career.
A second resource that Helgeland seems to have used is the 1950 film The Jackie Robinson Story. This film deals with such similarities as Jackie’s relationship with Rachel, the first meeting with Branch Rickey, and the incredible racism he faced in those beginning years. The Jackie Robinson Story also includes much of the biographical pre-baseball career information that 42 leaves out, most notably the relationship Jackie has with his brother Mack and his mother. Yet this film does not have as much of the subtle messages Helgeland included in 42 and seems to function more as a documentary than a biopic. It appears Helgeland used this source more as a reference for biographical information than a true source for his screen writing.