Shippen was born into a prominent Philadelphia family with Loyalist tendencies. She met Arnold during his tenure as military commander of the city following the British withdrawal in 1778. They were married in the Shippen townhouse on Fourth Street on April 8, 1779, and Arnold began conspiring with the British to change sides soon after. 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.