Ungerer was born in Strasbourg in Alsace, France, the youngest of four children to Alice (Essler) and Theo Ungerer. The family moved to Logelbach, near Colmar, after the death of Tomi's father, Theodore—an artist, engineer, and astronomical clock manufacturer—in 1936. Ungerer also lived through the German occupation of Alsace when the family home was requisitioned by the Wehrmacht.
As a young man, Ungerer was inspired by the illustrations appearing in The New Yorker magazine, particularly the work of Saul Steinberg. In 1957, the year after he moved to the U.S., Harper & Row published his first children's book, The Mellops Go Flying, and his second, The Mellops Go Diving for Treasure; by the early 1960s he had created at least ten children's picture books with Harper, plus a few others, and had illustrated some books by other writers. He also did illustration work for publications including The New York Times, Esquire, Life, Harper's Bazaar, The Village Voice, and for television during the 1960s, and began to create posters denouncing the Vietnam War.
Maurice Sendak called Moon Man (1966) "easily one of the best picture books in recent years." After Allumette: A Fable, subtitled With Due Respect to Hans Christian Andersen, the Grimm Brothers, and the Honorable Ambrose Bierce in 1974, he ceased writing children's books, focusing instead on adult-level books, many of which focused on sexuality. He eventually returned to children's literature with Flix 1998. Ungerer donated many of the manuscripts and artwork for his early children’s books to the Children’s Literature Research Collection at the Free Library of Philadelphia.
One consistent theme in Ungerer's illustrations is his support for European construction, beginning with Franco-German reconciliation in his home region of Alsace, and in particular European values of tolerance and diversity. In 2003, he was named Ambassador for Childhood and Education by the 47-nation Council of Europe.
In 2007, his home town dedicated a museum to him, the Musée Tomi Ungerer/Centre international de l’illustration.
Ungerer divided his time between Ireland, where he and his wife had moved in 1976, and Strasbourg. In addition to his work as a graphic artist and 'drawer', he was also a designer, toy collector and "archivist of human absurdity."
A biographical documentary film, Far Out Isn't Far Enough: The Tomi Ungerer Story, was produced in 2012. The film was featured at the 2013 Palm Springs International Film Festival. In 2015–2016, the Kunsthaus Zurich and the Museum Folkwang in Essen devoted a large exhibition to Ungerer's artistic oeuvre and in particular his collages. A comprehensive book has been published by Philipp Keel from Diogenes with essays by Tobias Burg, Cathérine Hug and Thérèse Willer.
Ungerer died on 9 February 2019 in Cork, Ireland, aged 87.