Childhood trauma may be prevalent in the general population, and the psychosocial treatment of patients with cancer may require consideration of the effects of such early adversity on the healing and recovery process. In this study, we investigated the long-term effects of childhood trauma in 133 women diagnosed with breast cancer (mean age 51, SD = 9) who had experienced physical, sexual, or emotional abuse or neglect. We examined their experience of loneliness and its associations with the severity of childhood trauma, ambivalence about emotional expression, and changes in self-concept during the cancer experience. In total, 29% reported experiencing physical or sexual abuse, and 86% reported neglect or emotional abuse. In addition, 35% of the sample reported loneliness of moderately high severity. Loneliness was directly influenced by the severity of childhood trauma and was directly and indirectly influenced by discrepancies in self-concept and emotional ambivalence. In conclusion, we found that childhood trauma was common in breast cancer patients, with 42% of female patients reporting childhood trauma, and that these early experiences continued to exert negative effects on social connection during the illness trajectory. Assessment of childhood adversity may be recommended as part of routine oncology care, and trauma-informed treatment approaches may improve the healing process in patients with breast cancer and a history of childhood maltreatment.