Brought up in Blundeston, Suffolk, on leaving school Mayhew studied at Lowestoft School of Art from 1982 to 1984, and then at Maidstone College of Art (now the University for the Creative Arts), graduating BA in 1987 with first class honours in illustration, followed by further studies in set design. He has published over 50 books. 
His first published work was Katie's Picture Show (1989), beginning a long-running series about a girl who explores famous paintings by climbing inside them. There are currently 13 titles in the series, including Katie's London Christmas. In 2014 Mayhew completely reillustrated the first title for a 25th anniversary edition.  A musical stage production of Katie and the Mona Lisa was successfully premiered at the Ashcroft Theatre, Croydon in April 2013. Katie in London was one of only 50 books chosen for the Books about Town project in 2014, where famous books were recreated as benches. Mayhew hand-painted his bench which was positioned near the Tower of London until auctioned off in October 2014.
Other books include the Ella Bella Ballerina series, Miranda the Castaway, BOY, illustrations for the "'Mouse and Mole" books (animated for BBC television, with the voices of Alan Bennett, Richard Briers and Imelda Staunton), "Koshka's Tales" (a collection of Russian Folk stories), Can you see a Little Bear? (illustrated by Jackie Morris) and Bubble and Squeak (with illustrations by Clara Vulliamy).
In 1994 he received The New York Times Award for one of the ten best illustrated books of the year for The Boy and the Cloth of Dreams, written by Jenny Koralek. Shakespeare's Storybook (a collaboration with Patrick Ryan) received an Aesop Accolade from the American Folklore Society in 2002. In 2011 James was the first illustrator selected to appear on the BBC's Authors Live series for Scottish Book Trust. Mayhew has had books published in Japan, China, Korea, Germany, France, Greece, Estonia, Spain, Scandinavia, America and many other countries.
More recently he has devised and presented classical music concerts for children with many different orchestras, incorporating narration and live illustration, usually projected. These have included "Peter and the Wolf", "The Firebird", "Swan Lake", "Pictures at an Exhibition", The Young Person's Guide to the Orchestra, and "Scheherazade". He has also participated in promenade performances at the Royal Opera House, Covent Garden. In 2013 he designed sets and costumes for a major production of the opera "Noye's Fludde" in Tewkesbury Abbey, to celebrate Benjamin Britten's centenary year, and as part of his role as the Guest Director of the 2013 Cheltenham Music Festival.
He is invited to speak at festivals, conferences and schools internationally and in 2014 was Illustrator in Residence at the Edinburgh International Book Festival. He has exhibited widely, notably at the Scottish National Gallery in 2010 and 2014/15. He teaches Children's Book Illustration to M.A. students at Anglia Ruskin University, and has taught courses on writing at the Arvon Foundation.
Books written or illustrated by Mayhew include
Katie and the Bathers
Katie and the Dinosaurs
Katie and the Starry Night
Katie and the Mona Lisa
Katie and the Sunflowers
Katie in London
Katie and the Impressionists
Katie's Picture Show
Katie and the Spanish Princess
Katie and the Waterlily Pond
Katie in Scotland
Katie and the British Artists
Katie's London Christmas
Who Wants a Dragon? (Illustrated by Lindsey Gardiner)
Koshka's Tales - stories from Russia
Ella Bella Ballerina and The Sleeping Beauty
Ella Bella Ballerina and Swan Lake
Ella Bella Ballerina and Cinderella
Ella Bella Ballerina and The Nutcracker
Madame Nightingale Will Sing Tonight
Miranda the Explorer
Miranda the Castaway
The Knight who Took All Day
To Sleep, Perchance to Dream (Shakespeare)
Shakespeare's stories (retold by Beverely Birch)
Barefoot Book of Stories from the Opera (retold by Shahrukh Husein)
Shakespeare's Story Book - Folk Tales that inspired the Bard (retold by Patrick Ryan)
Pinocchio (retold by Josephine Poole)
Mouse And Mole (by Joyce Dunbar)
Mouse And Mole have a Party (by Joyce Dunbar)
A very special Mouse And Mole (by Joyce Dunbar)
Happy Days for Mouse And Mole (Joyce Dunbar)
The Boy And The Cloth Of Dreams (by Jenny Koralek)
Tales of Ghostly Ghouls and Haunting Horrors (Written by Martin Waddell)
Boneless and the Tinker
Death and the Neighbours
Soft Butler's Ghost
- BBC Three Counties interview. 28 January 2005
- National Gallery - Children's picture books. Retrieved 2011-12-07.
- Katie and the Waterlily Pond: A Journey Through Five Magical Monet Masterpieces by James Mayhew - review. The Guardian. 29 April 2012. Retrieved 2012-09-20.
- http://www.wattspub.co.uk[dead link]
- James Mayhew: Authors Live. BBC Scotland. 12 May 2011. Retrieved 2011-12-07.
- James Mayhew's blog
- Mayhew on the BBC Three Counties website
- Mayhew on the Biography website
- Mayhew on the Scholastic website