He was born the son of the court noble Imakōji Motofuyu. His mother, daughter of an official serving Shōgo-in, later served Hino Nariko, wife of the shogun Ashikaga Yoshimitsu. Having lost his father early, he was raised by his brother and also became an adopted son of the regent Nijō Yoshimoto. These ties are thought to have opened the way for him to become Yoshimitsu's adopted son.
When the abbot of Sanbō-in was driven out after angering Yoshimitsu, Mansai took the tonsure and was at once appointed monzeki of Sanbō-in at Daigo-ji and head priest of Daigo-ji. He received a character from Yoshimitsu's name and called himself Mansai. It is said that rising to high rank before completing his training long remained a burden on his mind.
He accompanied Yoshimitsu, who had himself taken the tonsure, to Enryaku-ji on Mount Hiei and received the precepts together with him. Though he enjoyed special treatment as the shogun's adopted son, he also went step by step through the prescribed esoteric training, striving to acquire real ability as a monk.
Right after Yoshimitsu died, he was appointed a guardian priest who protected the shogun through prayer. It is said Yoshimitsu had not appointed him before because he knew Mansai's training was still incomplete. From then on he enjoyed the deep trust of the fourth shogun, Ashikaga Yoshimochi, and was consulted not only on prayers but also on politics.
He was appointed hōmu, overseeing the world of monks, and later rose to the rank of daisōjō. Also holding key posts such as chief abbot of Tō-ji, he became a representative monk of the Shingon school. The custom of the Sanbō-in abbot also serving as head priest of Daigo-ji is said to have begun with Mansai, a major turning point in the temple's history.
He began keeping a diary, noting each day's events in the margins of calendars, and continued it until the year of his death. This is the Mansai Jugō Nikki, which records in detail not only prayers but also shogunate councils, disputes among lords and diplomacy. His own handwritten copy survives and is regarded as a first-class source for the mid-Muromachi period.
By this year he had mastered all the esoteric teachings handed down at Sanbō-in. He studied and trained more than almost any monk of his day, even receiving teachings from monks of Mount Kōya. He is said to have overcome, through his own ability, the burden of having risen to high rank so young.
When Yoshimochi fell gravely ill without naming an heir, Mansai consulted the chief vassals and arranged for the next shogun to be chosen by drawing lots at Iwashimizu Hachiman-gū from among Yoshimochi's four brothers. Thus Ashikaga Yoshinori was chosen, the shogun by lottery. That same year, Mansai became the first Sanbō-in abbot granted the status of jugō.
Yoshinori was known for his harsh rule, but he is said to have listened carefully to Mansai's advice. Mansai mediated between the shogunate and the lords on difficult issues such as the conflict with the Kamakura kubō Ashikaga Mochiuji and disputes among the lords of Kyushu. Because he moved politics while being a monk, he is called the "prime minister in black robes".
This year he stepped down as head priest of Daigo-ji, a post he had held for nearly forty years. For governing the temple well over so long, he is honored, together with Gien who rebuilt it in the Azuchi-Momoyama period, as a restorer of Daigo-ji. Even as his health declined, the shogun kept consulting him.
Mansai died this year. Prince Fushimi-no-miya Sadafusa called him a "righteous man of the realm" in his diary, the Kanmon Nikki. It shows that his character, judging calmly while at the heart of the shogunate and showing consideration for others' positions, was highly valued by people of his own time.
Nijō YoshimotoAshikaga MochiujiAshikaga YoshimitsuAshikaga YoshimochiAshikaga Yoshinori
Shiba YoshimasaAshikaga YoshimitsuZeamiShō HashiAkamatsu MitsusukeAshikaga YoshimochiAshikaga YoshinoriIkkyū Sōjun
People of the Muromachi period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.