From 1931 to 1939 he was a well-known and respected painter in Italy, where he worked in the Futurism and avant-garde styles. In 1935 he received a doctorate in economics 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.
Children's author and illustrator
Lionni became the first children's author/illustrator to use collage as the main medium for his illustrations. Reviewers such as Booklist and School Library Journal have said that Lionni's illustrations are “bold, sumptuous collages” that include "playful patches of color" and that his “beautifully simple [and] “boldly graphic art [is] perfect to share with very young children.” Book World said that “the translucent color of the pictures and the simplicity of the text make a perfect combination.”
Leo Lionni would usually draw pictures as he told stories to his grandchildren, but one time he found himself on a long train ride with no drawing materials. Instead, he tore out circles of yellow and blue from a magazine to help him tell the story he had in mind. This experience led him to create his first book for children, Little Blue and Little Yellow (1959).
Lionni uses earth tones in his illustrations that are close to the actual colors of the objects found in nature. In his book Inch by Inch, for example, he uses realistic shades of brown and burnt orange in his collage of a robin, while the tree branches are shades of brown with dark green leaves. Mice are consistently found as characters in Lionni's books, such as the star character in Frederick and the title character in the Caldecott Honor Book Alexander and the Wind-Up Mouse. Lionni's illustrations have been compared to those of Eric Carle as both often employ animals, birds, insects, and other creatures to tell a story about what it is to be human.
Among Lionni's books that were not intended for children, the best known is probably Parallel Botany (1978; first published in Italian as La botanica parallela, 1977). This detailed treatise on plants that lack materiality—in other words, imaginary plants—is richly illustrated with drawings of plants in charcoal or pencil and photographs of 'parallel botanists'. The text is a rich mix of plant descriptions, travel tales, 'ancient' myths, and folk etymologies, leavened with historical facts and grounded in actual science. As an imaginary taxonomy, it is invoked by Italo Calvino as a precursor to the Codex Seraphinianus of Luigi Serafini.