Moss was born Thylias Rebecca Brasier, in a working-class family in Ohio. Her Native American father was a tire recapper, and her mother a maid. Moss has said that her father chose the name Thylias because he decided she needed a name that hadn’t existed before. According to Moss, her first few years of life were happy, with Moss and her family living in the upstairs rooms of an older Jewish couple named Feldman (who Moss believes were Holocaust survivors). The Feldmans treated Moss like a grandchild.
When Moss was five, the Feldmans sold their house and moved away. Her parents continued to live in the house with the new homeowners and their 13-year-old daughter, Lytta, who began to baby-sit Thylias after school. Lytta tormented Moss on a daily basis. In addition to this, as a child Moss experienced several horrific events, such as seeing a friend jump from a window to escape a would-be rapist and witnessing a boy on a bicycle get killed by a truck. "I never said a word of this to anybody," she later said. "I was there witnessing things that only happened when I left that house."
When Moss was nine her family relocated, causing her to be sent to school in a mostly white district. Treated badly by both her teachers and classmates for a number of reasons, some of them because of her race, she withdrew from social interaction at school and did not speak freely in classes until many years later in college. It was during this time she gave more attention to writing poetry, an activity she had begun two years earlier.