The reins of the HATAKEYAMA family were supposed to be given to Mochitoyo HATAKEYAMA (the younger brother of Mochikuni), who had been adopted into the family, but after Yoshihiro was born, Mochitoyo's succession was withdrawn in 1448 and Yoshihiro was designated the new successor after earning the official approval of Yoshimasa ASHIKAGA, the 8th Shogun of the Muromachi Shogunate.
畠山家の家督は養子に迎えた畠山持富（持国の弟）に決まっていたが、義就が出生すると48年に持富への相続を撤回し、室町幕府8代将軍の足利義政から裁可を得て義就が新たに後継者と定められる。