Mary Tourtel, née Caldwell, was born in Canterbury, England, a daughter of Samuel Caldwell, a stained-glass artist and stonemason on Canterbury Cathedral, and his wife Sarah. She was brought up in their artistic household. She studied art at the Sidney Cooper School of Art in Canterbury (now the University for the Creative Arts), and became an artist and children's book illustrator. She published her first book, A Horse Book and Three Little Foxes in 1897 at age 23. In 1900, she married Herbert Bird Tourtel, an assistant editor of The Daily Express.
The Express was in hot competition with The Daily Mail and The Daily Mirror, which then had popular illustrated characters called Teddy Tail and Pip, Squeak and Wilfred. The owner of the Express, Lord Beaverbrook, challenged Herbert Tourtel to come up with a character that would surpass its rivals, and he thought of his wife Mary. She created and illustrated Rupert Bear, who first appeared in a story called The Adventures of a Little Lost Bear in 1920. After 15 years of drawing Rupert Bear, she retired and Rupert Bear was continued by Alfred Bestall. Many books reprinting her stories were published in the 1920s and 1930s.