Duff was born on September 28, 1987, in Houston, Texas. Her parents are Robert Erhard Duff, a partner in a chain of convenience stores, and Susan Colleen (née Cobb), a homemaker turned film and music producer. Duff has one older sister named Haylie. She was raised between Houston and San Antonio, the locations of her father's convenience stores. Encouraged by their mother, both Hilary and her sister enrolled in acting, singing and ballet classes. The siblings earned roles in local theater productions, and later participated in a touring BalletMet production of The Nutcracker in San Antonio. Increasingly interested in the pursuit of show business, the Duff sisters and their mother moved to California in 1993, while their father stayed in Houston to take care of his business. The sisters auditioned for several years and were cast in many television commercials. Due to her acting career, Duff was home-schooled from the age of eight. The pair also modeled for various clothing brands. Duff stated, "My sister and I really showed an interest in [performing] and dedication, and [our mother] was like, 'How can I tell my kids no?' It's the same as kids that are going into sports. Parents support them and push them. "
In June 2016, Duff revealed that she has been working on a new album. In contrast to Breathe In. Breathe Out , she said that the new album has a "darker and heavier tone". She teased a new song via her Snapchat post, which was then confirmed to be titled "Tied to You".  However, no further news on the album has since been heard.