Born in Salisbury, Massachusetts, the daughter of a wealthy merchant, it is likely that Foster (née Webster) attended an academy for women like the one she described in The Boarding School; certainly, the literary allusions and historical facts that populate her work indicate an outstanding education.

In the 1770s she began writing political articles for Boston newspapers, and in 1785 she married a Dartmouth graduate, the Rev. John Foster. The two settled in Brighton, Massachusetts, where John Foster served as a pastor at First Church.

She bore six children, after which she wrote her two books and subsequently returned to newspaper writing. When her husband died in 1829, she moved to Montreal, Quebec, Canada, to be with her daughters. She died in Montreal, aged 81.

Her daughters Harriet Vaughan Cheney (born about 1815) and Eliza Lanesford Cushing were popular writers in the nineteenth century. Harriet Vaughan Cheney published Confessions of an Early Martyr (1840), A Peep at the Pilgrims in 1636 (Boston, 1850), The Rivals of Acadia and Sketches from the Life of Christ. Eliza Lanesford Cushing published Esther, a dramatic poem, and works for the young. The two sisters wrote in conjunction The Sunday-School, or Village Sketches.