Yoshioki, who had been determined to be a successor by his father when he was still young, was promoted to the rank of Jushii (Junior Fourth Rank) in Chikuzen Province in 1560, also appointed as Otomoshu and Shobanshu and permitted by the Muromachi bakufu (Japanese feudal government headed by a shogun) of the 13th shogun Yoshiteru ASHIKAGA to use the character '義' from his name, '足利義輝' and identified himself as '義長' (Yoshinaga).
早くから父に世子として指名され、永禄3年（1560年）には従四位筑前国に叙位・任官され、室町幕府第13代将軍・足利義輝から「義」の字を賜って義長と名乗り、御供衆・相伴衆に任じられる。