He was born into the Mibu (壬生) family in present-day Tochigi Prefecture, Japan and entered the Buddhist priesthood at Enryakuji (延暦寺) on Mt. Hiei (比叡山: Hieizan) near Kyoto at the age of 14.
In 838 A.D., his trip to Tang Dynasty China marked the beginning of a set of tribulations and adventures. Initially, he studied under two masters and then spent some time at Wutaishan (五臺山; Japanese: Godaisan), a mountain range famous for its numerous Buddhist temples in Shanxi Province in China. Later he went to Chang-an (Japanese: Choan), the then-capital of China, where he was ordained into both mandala rituals.
In 847 he returned to Japan and in 854, he became the chief priest of the Tendai sect at Enryakuji, where he built buildings to store the sutras and religious instruments he brought back from China.
He authored more than 100 books. His diary of travels in China (入唐求法巡礼行記, Nitto Guho Junrei Koki) was translated into English by Professor Edwin Reischauer under the title Ennin's Diary: The Record of a Pilgrimage to China in Search of the Law (Ronald Press, New York: 1955). Sometimes ranked among the best travelogues in world literature, it is a key source of information on life in Tang China.