At some point around 719, he became a subordinate of FUJIWARA no Umakai who was then Hitachi no kami (governor of Hitachi Province) and thereafter Mushimaro was under Umakai's patronage.
719年（養老3年）前後、藤原宇合が常陸守であった頃に宇合の下僚となり、以後宇合の庇護を受けたという。