Millie and Christine were born on July 11, 1851, to parents who were slaves on the plantation of Mr. Alexander McCoy. The plantation was near the town of Whiteville, North Carolina, which resulted in the girls also being referred to as The Carolina Twins. Prior to the sisters' birth, their mother had borne seven other children, five boys and two girls, all of ordinary size and form.
They were sold to a showman named Joseph Pearson Smith at birth, but were soon kidnapped by a rival showman. The kidnapper fled to the United Kingdom but was thwarted, since the United Kingdom had outlawed slavery in the 1830s.
Smith traveled to Britain to collect the girls and brought with him their mother, Monimia, from whom they had been separated. He and his wife provided the twins with an education and taught them to speak five languages, dance, play music, and sing. For the rest of the century, the twins enjoyed a successful career as "The Two-Headed Nightingale", and appeared with the Barnum circus. In 1869, a biography on the twins, titled History and Medical Description of the Two-Headed Girl, was sold during their public appearances.
- Waukesha Daily Freeman; Waukesha, Wisconsin; Thursday, June 29, 1882.
- Biographical Sketch of Millie Christine, the Carolina Twin, Surnamed the Two-Headed Nightingale and the Eighth Wonder of the World. Cincinnati: Hennegan & Co. Print, published between 1902 and 1912
- Fayetteville Observer; January 16, 2000.
- Fayetteville Observer; March 1, 2001.
- Millie-Christine: Fearfully and Wonderfully Made; Joanne Martell; John F. Blair Publisher (2001)
- Philadelphia Daily News; February 5, 2003.
- Biographical Sketch of Millie Christine, the Carolina Twin, Surnamed the Two-Headed Nightingale and the Eighth Wonder of the World. Cincinnati: Hennegan & Co. Print, [between 1902 and 1912].
- Millie and Christine McCoy bibliography
- American National Biography Online: Millie and Christine McCoy