It is recorded that, in 1523, at the Battle of Suga in Musashi Province, his father, Kageshige, was killed in action while Yasushige sustained injuries but survived to succeed his father as the head of the clan.  The Battle of Suga, however, occurred in 1455, so this is an error in the historical accounts of the Yokose clan.  The individual who died at the Battle of Suga was his great-grandfather, Yokose Sadakuni, while headship of the clan was inherited by his grandfather, Yokose Kunishige.

Yasushige served Iwamatsu Masazumi and wielded exclusive authority as the head of the chief retainers.  Consequently, Masazumi led a plot to expel him, but, instead, Yasushige killed Masazumi and backed Masazumi’s eldest son and designated heir, Iwamatsu Ujizumi, as the next head of the clan.  This enabled Yasushige to continue to wield his unchecked authority.  In 1545, Yashishige was killed at the Battle of Mibu in Shimotsuke Province.  He was sixty years old.  His eldest son and designated heir, Yokose Narishige (later known as Yura Narishige), succeeded him.

In the era of Yasushige, the Iwamatsu clan was usurped by the Yokose, becoming a puppet administration.  Meanwhile, the Yokose rose to the status of sengoku daimyō.  In his position as the head of the Yokose clan, Narishige killed Ujizumi and seized Nitta-Kanayama Castle.  The Yokose changed their surname to the Yura and became independent actors, and, as kokujin, or provincial landowners, in Kōzuke, played an important role in the history of the Kantō.