De Jong studied Dutch language and literature in the city of Amsterdam, in which she still lives. She was a teacher at several schools and worked as a bookstore clerk. Since she wanted to be a writer from childhood on, she published her first book, Het hondje (The Doggy), in 1978. De Jong has written over 25 books by now, for which she received several awards, including the international Janusz Korckzak Literary Award for Een verboden kind (A Forbidden Child).
Since she thinks too many children's books have nothing to do with reality, she feels the need to write good, realistic books that don't always have a happy ending.