From the Osaragi branch of the Hojo clan, descended from Hojo Yasutoki's younger brother Tomochika and based near Kamakura's great Buddha. Munenobu spent his early career serving in key shogunate offices including the hyojoshu and hikitsukeshu. He became the 10th shikken in 1311 following the death of ninth regent Sadatoki. By this time, however, real power had long been seized by the uchikanrei—Taira no Yoritsuna and Nagasaki Takasuke—and the regent's post was little more than a figurehead role. He resigned after only a few months, whether due to health or factional pressure remains unclear. He died in 1312. His brief tenure starkly illustrates how thoroughly the shikken office had been hollowed out as tokuso autocracy neared its peak.