Moore was born in Roswell, New Mexico. As a child, she had a difficult and unstable home life. Her parents split up before she was born, with her mother, Virginia, leaving Demi's biological father, Charles Harmon, Sr., and marrying Dan Guynes, a newspaper advertising salesman who frequently changed jobs; as a result, the family moved a great many times. Moore said in 1991, "My dad is Dan Guynes. He raised me. There is a man who would be considered my biological father who I don't really have a relationship with." Moore learned of him at age 13, when she found her parents' marriage certificate and inquired about circumstances since "I saw my parents were married in February 1963. I was born in '62." Moore was cross-eyed as a child; this was ultimately corrected by two surgeries. She also suffered from kidney dysfunction. She has a maternal half-brother, Morgan Guynes, who in 1988 was a U.S. Marine stationed in North Carolina, and gave her away at her wedding that year; a paternal half-brother, James Craig Harmon (born 1974) who in 2006 was sentenced to 10 years in jail for aggravated assault; a paternal half-sister, Charlotte Harmon Eggar, who in 2012 said she had not seen Moore in 30 years; a paternal half-brother, Charles Harmon Jr.; and a sister, who in 1998 was named Carolyn Sneed and living in Farmington, New Mexico.
Virginia Guynes left her husband when Moore was 15 and moved to West Hollywood, California, where Virginia worked for a magazine-distribution company. Two years later, Dan Guynes committed suicide. Moore attended Fairfax High School there, and recalled, "I moved out of my family's house when I was 16 and left high school in my junior year." She decided to pursue a career in the entertainment industry after being inspired by 17-year-old German actress Nastassja Kinski, who lived with her own divorced mother in the same apartment building. In August 1979, three months before her 17th birthday, Moore met musician Freddy Moore at the Los Angeles nightclub The Troubadour. Freddy Moore was married to another woman at that time. In February 1980, six months after they met, Freddy and Demi were married and lived in an apartment in West Hollywood.