Born in Lansdale, Pennsylvania, she lived in nearby Montgomery Square much of her life and came from a large and long-lived family, eight of her siblings dying in their 80s and 90s and one sister reaching 108.
By October 1987, when she was honored by the Pennsylvania legislature, she was recognized by Guinness as the oldest person in the United States, and the death of Anna Eliza Williams that December made her oldest authenticated person in the world. Her death shortly thereafter meant that she never appeared in a Guinness Book as oldest living person.
Her death caused some confusion as to who her successor was, with Guinness recognition and press publicity alighting first on Orpha Nusbaum (August 1875 - March 1988), who died before the 1989 edition's deadline, then Birdie May Vogt (August 1876 - July 1989), who appeared in the 1989 edition's main text, then Jeanne Calment, mentioned in the addenda section, and finally in November 1988 on Carrie C. White, whose claim to birth in November 1874 was accepted. However, with recent census research calling White's authentication into question, Jeanne Calment may very well have been Florence Knapp's actual immediate successor.