Langdon served as a member of the Second Continental Congress from 1775 to 1776. He resigned in June 1776 to become agent for the Continental forces against the British and superintended the construction of several warships including the "Raleigh", the "America", and the "Ranger", which was captained by John Paul Jones. In 1777, he equipped an expedition against the British, participating in the Battle of Bennington and commanding Langdon's Company of Light Horse Volunteers at Saratoga and in Rhode Island.