Hercules Mulligan established a tailoring business in New York and became involved in resistance to British imperial policy before the Revolutionary War. His shop served prosperous customers, including British officers, while his personal connections included the young Alexander Hamilton, who lodged with his family. Mulligan helped introduce Hamilton to local networks during the years when the latter was pursuing his education and entering public debate.

After British forces occupied New York in 1776, Mulligan remained in the city. His occupation offered opportunities to hear officers discuss military affairs and to observe preparations reflected in their clothing orders. He supplied intelligence to the American side while maintaining the appearance of an ordinary tradesman serving the garrison. This work depended on discretion, customers, and intermediaries.

Accounts of his activities associate an enslaved man called Cato with the dangerous work of carrying messages out of British-held territory. Cato's contribution should not be absorbed into a story of Mulligan acting alone. Later narratives credit their warnings with frustrating plans to capture George Washington, sometimes claiming that Washington's life was saved twice. The broad account of Mulligan's intelligence service is widely accepted, but the precise conversations, timing, and effects in these dramatic episodes are less securely documented than popular retellings suggest.

Mulligan continued tailoring after the war. Washington's association with him helped affirm his Patriot standing despite his wartime dealings with British clients. He joined the founders of the New York Manumission Society in 1785, participating in a campaign against slavery despite having himself enslaved Cato. That contradiction belongs in an account of his reform activity. He retired from business in 1820 and died in New York in 1825. His record leaves gaps about individual missions and Cato's work.