Born into an abolitionist family, Lydia Francis was primarily influenced in her education by her brother, a Unitarian clergyman and later a professor at the Harvard Divinity School. In the 1820s she taught, wrote historical novels, and founded a periodical for children, Juvenile Miscellany (1826). In 1828 she married David L. Child, an editor. After meeting the abolitionist William Lloyd Garrison (Garrison, William Lloyd) in 1831, she devoted her life to abolitionism.

Child's best-known work, An Appeal in Favor of That Class of Americans Called Africans (1833), related the history of slavery and denounced the inequality of education and employment for blacks; it was the first such work published in book form. As a result, Child was ostracized socially and her magazine failed in 1834. The book succeeded, however, in inducing many people to join the abolition movement. Child's further abolitionist efforts included editing the National Anti-Slavery Standard (1841–43) and later transcribing the recollections of slaves who had been freed.

In 1852 the Childs settled permanently on a farm in Wayland, Massachusetts. They continued to contribute liberally, from a small income, to the abolition movement. Child's other work included once-popular volumes of advice for women, such as The Frugal Housewife (1829), and books on behalf of the American Indian. Among her later books were three volumes of Flowers for Children (1844–47), Fact and Fiction (1846), The Freedmen's Book (1865), and An Appeal for the Indians (1868). Her letters have been compiled in Lydia Maria Child, Selected Letters, 1817–1880 (1982).