MacLean was born in Lincoln, Rhode Island to an Italian father and a British mother. MacLean's website reports that her mother was a spy, and worked for MI6. MacLean started reading romance because her older sister read the books, and she has wanted to be a romance novelist since she was a teenager. In 2000, MacLean received a B.A. in American Studies from Smith College in Northampton, Massachusetts. While at Smith, MacLean and her friends read hundreds of romance novels.

MacLean moved to New York City in 2000, and worked as a literary publicist until she attended graduate school at Harvard University, receiving a Masters in Education. Upon returning to New York City, she wrote her first book, a young adult novel,The Season after a friend suggested she try her hand at writing for teens. The book, set in Regency England, received numerous awards, and was named to the 2010 Lone Star Reading List of the Texas Library Association.

After The Season, MacLean wrote her first adult romance novel, Nine Rules to Break When Romancing a Rake, a Regency historical. The book debuted on the USA Today Bestseller List and the New York Times Bestseller List, where it stayed for four weeks, and was the first recipient of the Romantic Times Magazine Seal of Excellence.

MacLean and her husband live in Brooklyn, New York. She is extremely active in social media, and often discusses her daily life on twitter and facebook.