Rachel Foster was born in Pittsburgh, Pennsylvania, December 30, 1858 to Julia Manuel Foster and J. Heron Foster, the editor of the Pittsburgh Dispatch.
Elizabeth Cady Stanton, a prominent social activist and advocate of women's rights, held suffrage meetings at the Fosters' home, and Rachel's mother became vice president of the local suffrage society. After J. Heron Foster's death in 1868, Rachel, her sister, and mother moved to Philadelphia, where they joined the Citizens' Suffrage Association. Rachel began writing for newspapers at about age 17. At 21, she attended the eleventh convention of the National Woman Suffrage Association, and became actively involved in the organization by planning and organizing the association's meetings across the country in 1880 and 1881.
In 1882, she conducted the Nebraska Amendment campaign. Later, she published throughout the state of Pennsylvania the lecture of Governor John W. Hoyt of Wyoming, entitled "The Good Results of Thirteen Years' Experience of Women's Voting in Wyoming". In 1883, Rachel traveled through Europe with Susan B. Anthony.