Lionni's father was a Sephardic Jew who worked in the diamond business and his mother was an opera singer. He grew up in a house filled with his uncle's art collection. He lived within short walking distance of the two great museums in Amsterdam, the Rijksmuseum, and the Stedelijk Museum.

He married Nora Maffi, the daughter of the founder of the Italian Communist Party, and they had two sons, and later, grandchildren by both sons.

Leo Lionni died October 11, 1999, at his home in Tuscany, Italy, at the age of 89.

From 1931 to 1939 he was a well known and respected painter in Italy, where he worked in the Futurism and avant-garde style. In 1935 he received a doctorate degree in economics (1935) from the University of Genoa. During the later part of this period Lionni devoted himself more and more to advertising design.

In 1939 he moved to Philadelphia and began full time work in advertising, at which he was extremely successful, acquiring accounts from Ford Motors, and Chrysler Plymouth, among others. He commissioned art from Saul Steinberg, the then neophyte Andy Warhol, Alexander Calder, Willem de Kooning, and Fernand Léger.

In 1948 he accepted a position as art director for Fortune, which he held until 1960.

In 1960 he moved back to Italy, where he began his career as a children's book author and illustrator. Lionni produced more than 40 children's books. He received the 1984 American Institute of Graphic Arts (A.I.G.A.) Gold Medal and was a four-time Caldecott Honor Winner...for Inch by Inch (1961), Swimmy (1964), Frederick (1968), and Alexander and the Wind-Up Mouse (1970). He also won the Deutscher Jugendliteraturpreis in 1965.