Born in Gisborne, New Zealand, he is of Te Aitanga-a-Mahaki descent but also has close Maori tribal affiliations to the Ngai Tamanuhiri, Tühoe, Ngati Kahungunu, and Te Whanau-a-Apanui, and he has links to the Rongowhakaata, Ngati Porou, and Te Whakatohea. His original surname was Smiler which he keeps for day-to-day use. For writing he uses Ihimaera which is a Maori version of Ishmael, the first name of his great-grandfather.
For his schooling he spent he three years at Te Karaka DHS, a year at the Mormon Church College at Tuhikaramea, Hamilton, and he completed his University Entrance at Gisborne BHS. From 1963 to 1966 he attended Auckland University. However, he did not complete his degree and returned to Gisborne.
He began working as a cadet journalist with the Gisborne Herald, and then became a postman. In 1968 he moved to the Post Office in Wellington where he took a job as a journalist. Like many Maori at the time, Witi Ihimaera was not brought up speaking Maori but studied it at Victoria University of Wellington which he attended as a part-time student from 1969. He completed his BA in 1971. In 1970 he married, and he had two daughters.
An intensive six-month period of writing in London in 1970 resulted in the short story collection Pounamu Pounamu, published in 1972 and awarded third prize in the Wattie Book of the Year Awards, and the two novels Tangi, published in 1973 and awarded first prize in the Wattie Award, and Whanau, published in 1974.
His Pounamu, Pounamu, (1972), was the first collection of short stories to be published by a Maori writer. It was read by the Prime Minister and Witi Ihimaera was soon seconded to the Ministry of Foreign Affairs. He later became a career diplomat and during this time he worked for the New Zealand High Commission in Canberra, Australia, and from 1986 to 1989 he was in the United States. For 1988 to 1989 he was the New Zealand Consul in New York.
In 1975 Witi Ihimaera received the Robert Burns Fellowship in creative writing at the University of Otago. Another period of intensive writing resulted in The New Net Goes Fishing, published in 1977, and he began editing the anthology Into the World of Light, published in 1982.
However, in December 1975 he decided to move away from his initial purpose to "establish and describe the emotional landscape of the Maori people" and to consider more the political and social reality. After a ten-year break from writing he produced The Matriarch, published in 1986, and which received the Wattie Book of the Year Award. He also wrote the libretto for an opera by Ross Harris based on Whanau.
He was then appointed Distinguished Creative Fellow in Maori Literature at the University of Auckland, and he has taught at the English Department there since 1990. In 1990 he was awarded a New Zealand Scholarship in Letters.
Also in 1990 he wrote the first draft of Nights in the Gardens of Spain which was semi-autobiographical and described the coming-out of a married father of two daughters. This was to be his own coming out and the start of his writing on sexuality. However, out consideration for the feelings of his daughters, he did not publish the final version until 1995.
His novel Bulibasha: King of the Gypsies, (1994), was awarded the 1995 Montana Book Award. He is a member of the Te Haa, a Maori writing committee and a publisher in a Maori writing series. He is a founder member of Te Waka Awhina Tane, a support group for gay Maori men and women.