The daguerreotype shows a 7-year old girl. Her face is pale, her expression somber. Her elegant plaid dress, trimmed in lace, and the notebook on the cloth-covered table behind her, suggest that she comes from a prosperous family.

Though modest, the photograph taken in Boston in 1855, is actually historic. It shows not a white child but a black girl — Mary Mildred Williams — who was born into slavery. It was an image so compelling to white Americans at the time that it helped transform the abolition movement. Housed in relative obscurity at the Massachusetts Historical Society, the daguerreotype was recently rediscovered by the photographer and scholar Jessie Morgan-Owens while researching her dissertation.

“Mary’s daguerreotype was one of the first images of photographic propaganda and one of the first portraits made solely to prove a political point,” Ms. Morgan-Owens wrote. “It marks a forgotten moment in media history: when photography, introduced to the United States in 1839, first began to make its tenacious claim on our sympathies and on our political points of view.”

Mary is now the subject of Ms. Morgan-Owens’s groundbreaking book, “Girl in Black and White: The Story of Mary Mildred Williams and the Abolition Movement” (W.W. Norton & Company). Mary and her family had been enslaved in Virginia. Her father, Henry Williams, escaped to Boston and worked with members of the Vigilance Committee, which provided legal and financial help for fugitive slaves, to free his family in the South. Senator Charles Sumner of Massachusetts, a prominent ally, saw an opportunity in the family’s story.