Mij was born in Edinburgh. She studied English at York University, Publishing Studies at Stirling University and has worked in a variety of jobs - as a lexicographer, oral historian, editor of a community newspaper and, most recently, as a specialist journalist in artificial intelligence.
Her children's novel 48 Hours with Franklin (Blackie and Puffin, 1993) won the Kathleen Fidler Award in 1993. More recently, her picture book text William and the Night Train (illustrated by Alison Jay) was included in The White Ravens 2002, the International Youth Library's (Internationale Jugend Bibliothek) annual selection of outstanding picture books for children. Forthcoming titles include Where's My Darling Daughter? (OUP).and One Sheep More (Hodder).
Mij lives in Yorkshire and has two children.