Mac Barnett is an American children's book writer best known for his advocacy of the picture book. He has published over seventy works since 2009, consisting of over fifty picture books, graphic novels, essays, and short stories. He currently serves as the National Ambassador for Young People's Literature (2025-26).

Barnett graduated from Pomona College, where he studied under the writer David Foster Wallace.

Extra Yarn, a picture book illustrated by Jon Klassen, won the 2012 Boston Globe–Horn Book Award and 2013 E. B. White Read Aloud Award. It was a Caldecott Medal Honor Book. Sam & Dave Dig a Hole, illustrated by Jon Klassen, won a Caldecott Honor and the 2015 E.B. White Read Aloud Award.

He is the co-creator, with Jon Klassen, of the Emmy-award winning children's animated series Shape Island on Apple TV+, based on their Shapes trilogy of picture books.

In 2011, Barnett wrote the Picture Book Proclamation, an art manifesto about picture books signed by 21 other children's writers.

In 2024, Barnett wrote La Porta Segreta, a book for adults about children's books, published in Italian by Terre di Mezzo.

In February 2025, Barnett was inaugurated at the Library of Congress as the 2025-6 National Ambassador for Young People’s Literature.