A large, round-faced man who could sling the blarney as well as anyone, Mulligan opened his first shop on Water Street, near Lower Manhattan's busy East River wharves. Mulligan later relocated to 23 Queen Street (today's Pearl Street). His customers included wealthy British businessmen and high-ranking military officers. What those fine gentlemen weren't aware of was that the genial Irish proprietor was one of the most ardent patriots in the colonies.

Mulligan took up the patriot cause at least a decade before the start of the Revolutionary War. In 1765, he joined the Sons of Liberty, an underground group that engaged in anti-British agitation.

In January 1770, he fought in the Battle of Golden Hill, a skirmish with British soldiers that was one of the first violent incidents leading up to the Revolution. In the summer of 1775, he helped the Sons of Liberty steal a cache of muskets from the city armory. Mulligan was also a member of the New York Committee of Correspondence, a group that rallied opposition to the British through written communications.