Peggy played a role in the conspiracy which was exposed after British Major John André was arrested in September 1780 carrying documents concerning the planned surrender of the critical Continental Army base at West Point.

[3] The British captured Philadelphia in September 1777, and the Shippen family held social gatherings at their home, in keeping with their political interests and stations.

In 1779, the Supreme Executive Council of Pennsylvania had brought eight formal charges against Arnold for corruption and malfeasance with the money of the federal and state governments, and he was subsequently convicted on two relatively minor counts.

[7] Arnold purchased Mount Pleasant on March 22, 1779, a manor home built in 1762 for Captain John Macpherson, and he deeded the property to Peggy and any future children.

The couple honeymooned at family homes in New Jersey and Pennsylvania, then returned to Philadelphia to take residence at Arnold's military headquarters in the Masters-Penn mansion.

Pursuant to the secret communications with the British, he sought and obtained the command of West Point, a critical American defense post in the highlands of the Hudson River.

Peggy and their infant son Edward Shippen Arnold (born 19 Mar 1780) joined him there in a home on the Hudson two miles south of West Point.

On Thursday, September 21, 1780, General Arnold met with André on the shores of the Hudson River and gave him documents and maps about the fortifications at West Point in anticipation of the British capture of that site.

Peggy's dramatics—described by the author Nancy Rubin Stuart as "the grandest theatrical performance of her life"—completely fooled Washington and his aide Alexander Hamilton, who believed her innocent.

In November 1780, her father escorted Peggy and her infant son to the shores of the Hudson where she boarded a boat to New York City to join Arnold.

[13] Hostilities appeared to be winding down in North America after Cornwallis' surrender at Yorktown in October 1781, and the Arnolds left for London on December 15, 1781—including their second child James Robertson (born in August)—arriving January 22, 1782.

[14] Peggy was initially welcomed warmly in Great Britain, as was her husband; she was presented at English court to the queen on February 10, 1782, by Lady Amherst.

Arnold left for a business opportunity in 1784 and sent to Connecticut for his three sons Benedict, Richard, and Henry (by his first wife) to join him in Saint John, New Brunswick.

James Parton, a biographer of Aaron Burr, published an account in the 19th century, after all of the principal actors had died, implying that Peggy Shippen Arnold had manipulated or persuaded Benedict to change sides.

[19] According to Burr's notes, Shippen Arnold "was disgusted with the American cause" and "through unceasing perseverance, she had ultimately brought the general into an arrangement to surrender West Point".

They claimed that Burr rode with Peggy Shippen Arnold in the carriage to Philadelphia after her stay with Mrs. Prevost, and that he fabricated the allegation because she refused advances that he made during the ride.

Shippen is also the subject of at least four historical novels: The Exquisite Siren by E. Irvine Haines (1938), Peggy by Lois Duncan (1970), Finishing Becca by Ann Rinaldi (1994), and The Traitor's Wife by Allison Pataki (2014).