She is of English, Scottish, Irish, Welsh, German, and Swedish descent.
Roberts was born on October 28, 1967, in Smyrna, Georgia, to Betty Lou Bredemus (1934–2015) and Walter Grady Roberts (1933–1977). Her older brother, Eric Roberts, from whom she was estranged for several years until 2004, sister Lisa Roberts Gillan, and niece Emma Roberts, are also actors.
Her parents married in 1955. Roberts' parents, one-time actors and playwrights, met while performing in theatrical productions for the armed forces. They later co-founded the Atlanta Actors and Writers Workshop in Atlanta, off Juniper Street in Midtown. They ran a children's acting school in Decatur, Georgia, while they were expecting Julia. The children of Martin Luther King, Jr. and Coretta Scott King attended the school; Walter Roberts was their daughter Yolanda Denise King's acting coach. As a thank-you for his service, Mrs. King paid Mrs. Roberts's hospital bill when Julia was born. Her mother filed for divorce in 1971; the divorce was finalized in early 1972.
In 1972, her mother married Michael Motes, who was abusive and often unemployed; Roberts despised him. The couple had a daughter, Nancy Motes, who died at 37 on February 9, 2014, of an apparent drug overdose. The marriage ended in 1983, with Betty Lou divorcing Motes on cruelty grounds; she had stated that marrying him was the biggest mistake of her life. Roberts's own father died of cancer when she was ten.