After joining the Buddhist priesthood at Gango-ji Temple in Nara, he became a disciple of Saicho. In 810, together with Saicho, he created the Three Precepts of Buddhist law on Mt. Hiei, and established rules for dwelling at temples. In July and August 812, as a result of Saicho having fallen ill, he was appointed the head priest of all of Mt. Hiei, but internal strife among the clergy of the mountain (Taihan irritated the clergy with his personal inconveniences, so he asked Saicho for a leave of absence, and permission to depart from the mountain) caused him to seclude himself from the world on Takashima island in Omi Province.
Saicho took Taihan with him to go twice to visit Kukai and persuade him to bestow the dharmic succession ritual of the Diamond Realm on him, but Taihan refused (this led Saicho to go in 812 without Taihan, accompanied by other disciples, to see Kukai, who then granted him the dharmic succession ritual of the Diamond Realm). On January 23, 813, Taihan, along with Saicho and others, did receive the succession ritual of the Womb Realm from Kukai at Takaosan-ji Temple (Jingo-ji Temple), and thereafter became a disciple of Kukai. On April 14, 813, he also received the succession ritual the Diamond Realm.
Thereafter, Saicho--recognized as Kukai's favorite pupil, and groomed as his successor--tried to press Taihan into returning over and over to Mt. Hiei, but Taihan did not in fact return. In the end, it is said that Kukai had Taihan write a letter and sent it off to Saicho.
This struggle over Taihan, as well as Saicho's views on esoteric Buddhist ritual, among other factors, eventually led to a separation between Saicho and Kukai. Nonetheless, it is said that to the very end Saicho never gave up his quest to have Taihan return to Mt. Hiei.
In 816, Taihan established temporary residence at Otokuni-dera temple, and when Kukai established the monastery of Mt. Koya, Taihan, along with Kukai's other disciple Jichie, worked hard, climbing the mountain and erecting a thatched hut.
In 837, when Taihan would have been about 60, his name appears on a document of Buddhist precepts as one of the established monks of To-ji Temple, but thereafter nothing is known of what became of him. Taihan was one of the Ten Great Disciples of Kukai, and is counted as one of the Four Sages, but because very little is known of his priestly career, he is frequently considered as on a par with Kojo, another disciple of Saicho's, but this comparison is considered an error.