In 1920, Foster founded the Negro National League, the first stable Negro League, primarily for purposes of booking games for the American Giants. With a stable schedule and reasonably solvent opponents, Foster was able to improve receipts at the gate. When opposing clubs lost money, he would often help them meet payroll, sometimes out of his own pocket. (This was in his club's self-interest, of course, but also stemmed from Foster's belief that African-American ballplayers needed a stable league of their own.)
Foster ran the American Giants and the NNL with a firm hand (some say too firm), until he fell ill in 1926. After a lingering illness, he died in 1930. After his death, without an equally firm hand at the tiller, the league folded, but was succeeded in 1933 by a new Negro National League.