Margaret “Peggy” Shippen was born to Edward Shippen IV and Margaret Francis in Philadelphia, Pennsylvania in 1760. The Shippen's were among Philadelphia’s most prominent families, although their loyalties were divided with the coming of the American Revolution. Edward, a prominent Philadelphia merchant and judge, held moderate tory sympathies but remained outwardly neutral when the war broke out, while her elder sister Elizabeth married a colonel in the Continental Army.

Peggy’s own political views became obvious during the British occupation of Philadelphia. Peggy met a British officer by the name of John Andre, whose sketch of the young lady provides a glimpse into her life and personality. Peggy would eventually conspire with Andre at the time of her husband, Benedict Arnold’s, treason later in the war. Although it has been speculated that Peggy pursued a romance with Andre during the British occupation of Philadelphia, little evidence exists to support the claim, aside from a sketch that Andre made of Peggy. Regardless, the two kept up a correspondence after the British abandoned the Philadelphia.

On April 8, 1779, Peggy married Arnold, at age nineteen. Shortly thereafter, Arnold made his fateful decision to defect to the British. Though it is unclear whether she instigated his defection, Peggy was complicit in the execution of Arnold’s treason and may have been the one to introduce her husband to Andre. Peggy had remained in Philadelphia, where she was able to pass information from her husband’s letters through a chain of communication and coded letters that ended with John Andre.

During the climactic episode in which Arnold’s defection was revealed to the Continental Army, Peggy feigned hysteria to avert suspicion about her involvement in the conspiracy while also helping to delay Arnold’s capture by Patriot forces and flee the Continental Army. Shippen joined him in British-held New York City after the city council in Philadelphia banished her.

After the British surrender at Yorktown, the Arnolds moved to London, whose high society welcomed Peggy with open arms. Queen Charlotte awarded her an annual pension of £100 to care for her children, and King George himself presented her with a gift of £350 for her services. Her hometown, on the other hand, had no such fondness for her, despite her father’s continued influence. She received a frightfully cold reception when she visited her family one last time in 1791, and even faced some protests until she departed. Meanwhile, Benedict continuously labored to build his fortune in either London or New Brunswick in Canada, to no avail—he only left her a collection of debts when he died. Peggy herself passed away in 1804, and was buried in St. Mary’s Church in Battersea.