In 1765, Mulligan became one of the first colonists to join the Sons of Liberty, a secret society formed to protect the rights of the colonists and to fight British taxation. In 1770, he helped to mob British soldiers in the Battle of Golden Hill. He was a member of the New York Committee of Correspondence, a group that rallied opposition to the British through written communications. In August 1775, he and the Corsicans, a New York volunteer militia company, while under fire from HMS "Asia", captured four British cannons in the Battery. In 1776, Mulligan and the Sons of Liberty knocked down a statue of King George III in Bowling Green and then melted the lead to cast bullets to use against the British. Mulligan remained in New York as a civilian after Washington's army was driven out during the New York campaign in summer 1776.