Jameson was transferred April 7, 1777, to the 3rd Continental Light Dragoons, where he fought at the Battle of Brandywine Creek later that year.

He engaged the British at the New Jersey Battle of Monmouth Court House, in June 1778, and was promoted to the rank of colonel in August 1779.

In 1780, General George Washington placed key commanders in strategic areas around West Point, New York.

Soon afterward, a man calling himself "John Anderson" was intercepted along a Hudson River valley footpath by militiamen and found to be in possession of documents that included information regarding the defenses of West Point, as well as movements of the American army.

Since the papers were found in an odd place, "under the feet of his stockings", Colonel Jameson became alarmed and had Anderson detained.

Jameson sent word to—and alerted—General Arnold, even though he had been dissuaded to do so by Major Benjamin Tallmadge, head of Continental Army Intelligence.

The investigation further revealed that Benedict Arnold, as commandant of West Point, had agreed to surrender the fort to the enemy in return for a royal commission in the British army and a large sum of money.