His secular name was Morito ENDO. Being originally a samurai belonging to the Watanabe party in Settsu Province, which was under the umbrella of Settsu-Genji (Minamoto clan), he served Imperial Princess Muneko (Josaimonin), a daughter of Emperor Toba, as a Hokumen samurai (samurai on guard at the ex-emperor's palace). He is said to have loved Kesa Gozen, the wife of Wataru WATANABE, who was his male cousin and worked together with him, killed her by mistake, and consequently become a priest.
(Based on this incident, the nagauta (ballad sung to samisen accompaniment, sometimes with other instruments) "Toba no Koizuka" (tomb of the lover in Toba) was created in the Meiji period.)
Wishing to revive Jingo-ji Temple, where priests were absolutely corrupt, Mongaku presented a direct petition to Emperor Goshirakawa, which resulted in Mongaku being exiled to Izu Province, which was the feudal domain of MINAMOTO no Yorimasa of Settsu-Genji (Minamoto clan), the head of the Watanabe party. Mongaku persuaded MINAMOTO no Yoritomo, who was also exiled to Izu Province, to take up arms against the Taira family. Thereafter, he restored temples in various places under the patronage of Yoritomo and the Cloistered Emperor Goshirakawa. After Yoritomo's death, however, Mongaku got on the wrong side of Emperor Gotoba, and was then exiled to Sado Province and died there.
"Jigokumon" (Gates of Hell), a film directed by Teinosuke KINUGASA that won the Grand Prix at the Cannes Film Festival, depicts a drama where Morito ENDO (whose role was played by Kazuo HASEGAWA) loved Kesa Gozen (whose role was played by Machiko KYO), the wife of Wataru WATANABE (whose role was played by Isao YAMAGATA), and consequently became a priest.
"Hinotori (the Phoenix): Turbulent Times" (cartoon) by Osamu TEZUKA depicts Morito ENDO and Mongaku as different persons.