DeVillers attended Colonie High School in Albany, New York. She studied Communications at the State University of New York, Oswego and obtained an M.A. in Journalism from The Ohio State University. She has a twin sister, Jennifer Roy, who is six minutes older. Jennifer also helped Julia write Trading Faces and Take Two. Her father is German.

DeVillers lives in New Albany, Ohio, with her husband and two children.

Her latest book is Liberty Porter, First Daughter which is about a 9-year-old girl whose father is elected president and moves into the White House. DeVillers teamed up with Disney Channel star, Sabrina Bryan to write the book, Princess of Gossip. She also wrote Girlwise: How to Be Confident, Capable, Cool, and In Control and The College Dorm Survival Guide: How to Survive and Thrive in Your New Home Away from Home. She also writes the Tuned In series, written for pre-teen girls.

Another book for tweens, Trading Faces, is coauthored with her twin sister Jennifer Roy (author of Yellow Star). It was published January 2009. Her upcoming books include a young adult novel, LYNNVisible.

Her third book is based on her experiences at SUNY-Oswego.

When DeVillers was younger she created a list of things she wanted to do when she was older. This list included things like "be published", "have a cute boyfriend", and others. This list turned out to be the inspiration for How My Private, Personal Journal Became a Bestseller.