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 J. R. R. 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. Years later she provided some new illustrations for The Land of Narnia: Brian Sibley Explores the World of C. S. Lewis (1998).
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[a] 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 of Ham 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 as well as the Tolkien-related A Map of Middle-earth.
Baynes listed as one of her favourites the artwork she provided for A Dictionary of Chivalry (1968), a project that required two years to complete as it contained nearly 600 illustrations. For this work she won the Kate Greenaway Medal from the Chartered Institute of Library and Information Professionals (CILIP) for the year's best children's book illustration.
Four years later, Baynes was a commended runner-up for the Greenaway, for Snail and Caterpillar by Helen Piers.
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 also provided the covers for a Puffin Books edition of the entire series issued in the 1980s.
Baynes married German-born Fritz Otto Gash in 1961, and they lived in a village near Farnham until his death in 1988. Their only child was still-born. Apart from her art, Baynes' interests included world religions and cultures, her pet dogs, and the music of Handel, to which she listened while working. According to her obituaries, she had a warm relationship with Tolkien but was professionally offended when learning of C. S. Lewis's criticism "that she could not draw lions".