Pauline Baynes was born in Hove, Sussex. For a few years she was raised in India, where her father was commissioner in Agra, but she and her elder sister were sent back to England for their schooling. She spent much of her childhood in Farnham, studying at the Farnham School of Art (now the University for the Creative Arts and eventually attended the Slade School of Fine Art, but after a year there she volunteered to work for the Ministry of Defence, where she made demonstration models for instruction courses. This work did not last long. She was soon transferred to a map-making department, where she acquired skills that she later employed when she drew maps of Narnia for Lewis and of Middle-earth for Tolkien.
Baynes is probably best known for her covers and interior illustrations for The Chronicles of Narnia by C. S. Lewis, seven books published, one volume a year, from 1950 to 1956 (the first five by Geoffrey Bles, the last two by The Bodley Head). Years later she provided some new illustrations for The Land of Narnia: Brian Sibley Explores the World of C. S. Lewis (HarperCollins, 1998), by Brian Sibley. (According to a School Library Journal review, "the artwork includes full-page illustrations in glowing color".) When she began work on the Narnia books she was already the chosen illustrator of Lewis's friend and colleague J. R. R. Tolkien.
In her obituary for The Daily Telegraph Charlotte Cory described how Baynes and Tolkien came to be associated: In 1948 Tolkien was visiting his publishers, George Allen & Unwin, to discuss some disappointing artwork that they had commissioned for his novella Farmer Giles of Ham, when he spotted, lying on a desk, some witty reinterpretations of medieval marginalia from the Luttrell Psalter that greatly appealed to him. These, it turned out, had been sent to the publishers "on spec" by the then-unknown Pauline Baynes. Tolkien demanded that the creator of these drawings be set to work illustrating Farmer Giles of Ham and was delighted with the subsequent results, declaring that Pauline Baynes had "reduced my text to a commentary on her drawings". Further collaboration between Tolkien and his Farmer Giles illustrator followed, and a lifelong friendship developed ... Later, when she showed him her artwork for a poster featuring Frodo and Bilbo Baggins, the author nodded approvingly and murmured quietly: "There they are, there they are." Eventually drawings by Baynes appeared not only in Farmer Giles of Ham, but also in The Adventures of Tom Bombadil, Smith of Wootton Major, Tree and Leaf and (after the author's death) the poem Bilbo's Last Song, which appeared as a poster in 1974 and as a book in 1990.
Baynes also painted the covers for two British paperback editions of The Lord of the Rings (in one volume in 1973 and in three volumes in 1981) and produced illustrated poster versions of the maps from The Lord of the Rings and The Hobbit. However, Baynes's own favourite among her works was the set of illustrations she provided for A Dictionary of Chivalry, edited by Grant Uden (Longman, 1968), a project that required two years to complete. As a result, she won the Kate Greenaway Medal from the Library Association for the year's best children's book illustration by a British subject. In a retrospective citation, the Library Association calls it "a reference work that details the life and thoughts of knights". As a reference book it is unique among the winning works and only one other Greenaway Medal in almost sixty years has been awarded for the illustration of non-fiction.
Four years later, Baynes was a commended runner-up for the Greenaway, for Snail and Caterpillar by Helen Piers (Longman, 1972). Baynes also illustrated The Borrowers Avenged by Mary Norton (1982), the fifth and final book in the Borrowers series, following the death of Diana Stanley, who had illustrated the previous four books. Baynes did the covers for a Puffin edition of the entire series issued in the 1980s.