Sandra Louise Smith was born on May 28, 1944, the daughter of New York City native Raymond Smith, then serving in the military, and Pauline Bayne, a pencil factory worker from Huntsville, Alabama, who was of mostly Scottish descent, with matrilineages in South Carolina dating back to the late 18th century. Locke's parents separated before her birth. In her autobiography, Locke noted that "although Momma would not admit it, I knew Mr. Smith never married my mother." She had a maternal half-brother, Donald (born April 26, 1946) from Bayne's subsequent brief marriage to William B. Elkins. When Bayne married Alfred Locke in 1948, Sandra and Donald adopted his surname. She grew up in Shelbyville, Tennessee, where her stepfather owned a construction company; the family later moved to nearby Wartrace. Self-described as introspective and ambitious, Locke started working part-time at age 16, drove her own car, and had a telephone installed in her bedroom.