Tallmadge was a major in the 2nd Continental Light Dragoons. He was initially commissioned on June 20, 1776. Eventually, as the chief intelligence officer for George Washington, he was promoted to the rank of colonel. He organized the Culper Spy Ring based out of New York City and Long Island during the American Revolutionary War, using the code name John Bolton. There is actually very little evidence to prove that Tallmadge had heard from a spy in New York City about the Arnold-André plot. However, it would have been easy for Tallmadge to suspect that Arnold was up to no good, since Arnold had arranged to meet Anderson (Major John Andre's alias at the time) and Anderson was carrying military secrets back to New York City. The only thing Tallmadge could do was to persuade Jameson to recall lieutenant Allen who was already on his way to deliver the prisoner André into Arnold's custody. However, Tallmadge was unable to dissuade Jameson from informing Arnold of Major André's arrest. Tallmadge's suspicion of Arnold's treachery may not have been strong enough as Jameson later reported in a letter to Washington that neither Tallmadge nor other officers he consulted raised any objections to sending lieutenant Allen with a message to Arnold saying André was now in Jameson's custody.
After Benedict Arnold's British contact, John André, was caught, he was taken to North Castle, where the commander, Colonel Jameson, ordered his Lieutenant, Allen, to take a note and the incriminating documents found with André to their commander, Benedict Arnold, at West Point. Tallmadge, suspecting André to be a spy, and Benedict Arnold to be his accomplice, tried to have Jameson reverse his orders. He was unsuccessful, but did convince Jameson to send a rider and take Andre to Salem, eight miles east of the Hudson River and to send the documents to George Washington. Lt. Allen was still to report to Benedict Arnold with Jameson's note outlining the events. Later, Jameson was chastised by Washington for warning Arnold and allowing his escape. André was placed in Tallmadge's custody until André's execution.