Frances Adeline Seward (called “Fanny” among family and close friends) was born on December 4, 1844, to William Henry Seward and Frances Adeline (Miller) Seward, her namesake. Fanny was the youngest of five children. Her siblings included Augustus Henry Seward, Frederick William Seward, Cornelia Frances Seward (who died in infancy), and William Henry Seward, Jr. Frances was the only surviving daughter of William and Frances Seward.

As a young girl, Fanny contracted typhoid fever which left her in poor health for the rest of her life, commonly plagued with sore throats, coughs, and fevers. Frances was an aspiring writer, encouraged by both of her parents to read extensively and practice writing poetry and plays while also keeping a detailed diary, which Fanny began at the age of 14 in 1858. When her father was appointed the Secretary of State in 1861, Fanny accompanied her father to-and-from Washington D.C., documenting political life during the Civil War. Fanny’s detailed journal entries also record her father's carriage accident on April 5, 1865, and the subsequent attack on the Seward home and aborted assassination attempt by Lewis Powell a little over a week later on April 14.    

Even though Powell injured many members of the Seward family, including Fanny, all survived the attack. Her mother, however, died soon after in June 1865. Fanny, already frail, died on October 29, 1866, likely from tuberculosis, weeks before her twenty-second birthday.