Born on September 16, 1948, in England, Nilsen later attended art school and graduated with a diploma in art and design from Edinburgh College of Art. She then earned her Scottish teaching certificate from Moray House College of Education before working as an assistant art teacher and then a course director in natural history illustration. All of her studies and teaching further sparked her creativity, and an editor at Walker Books approached Nilsen asking her to create a new idea for a puzzle book. Not yet giving up her teaching, though, Nilsen's ingenuity gave her employment with Walker Books exclusively for five years. In 1995, Nilsen decided to work on children's books full-time. Over all, she has authored and illustrated over fifty children's books, including her mystery-art books the Art Fraud Detective and the Great Art Scandal. Nilsen also creates puzzles, games, and CD-ROM programs, and she has worked for LEGO, the National Gallery London and Scotland, and the Chicago Art Institute. Educational and fun, Nilsen's works have been translated into eighteen languages. Not lacking in ideas, Nilsen continues creating interactive learning materials for children.