He was born in Brooklyn, New York, the son of Irish immigrants. He studied at the Pratt Institute and the Art Students League of New York. He worked as an illustrator for pulp magazines such as *Weird Tales* and *Astounding Science Fiction*, as well as for book publishers such as Ace Books and Ballantine Books. He also illustrated covers for The New York Times Book Review and other newspapers and magazines.
Sinnott's work was influenced by the illustrators Virgil Finlay and Hannes Bok, and he is considered one of the most important science fiction illustrators of the 20th century. His illustrations are characterized by their realism, detail, and sense of wonder. He was inducted into the Science Fiction Hall of Fame in 2009.
Sinnott's work has also been exhibited in galleries and museums, and he has received numerous awards, including the World Fantasy Award for Best Artist in 1978, and the Hugo Award for Best Original Artwork in 1990.
He died in New York City in 2009.
Some of Sinnott's most well-known works include:
* Covers for *Astounding Science Fiction*, *Analog Science Fiction and Fact*, *The Magazine of Fantasy and Science Fiction*, and *Galaxy Science Fiction*.
* Illustrations for _The Chronicles of Narnia_, _The Lord of the Rings_, and _Dune_
* Cover art for _The Foundation Trilogy_, _The Hitchhiker's Guide to the Galaxy_, and _The Ender Saga_.
* Concept art for the films *Star Trek: The Motion Picture* and *Dune*.