Born in Coleraine Ireland, he emigrated to New York City with his family at six years old. After graduating from King’s College, Mulligan married Elizabeth Sanders, the niece of Admiral Sanders of the Royal Navy. Quite irrespective of his wife’s British background, Mulligan was a staunch patriot. During the 1770s, Mulligan lived with his close friend Alexander Hamilton and was able to impart some of his flag-waving notions onto the young scholar. Often engaging in late night discussions, the two colleagues exchanged ideas in such a way that Hamilton’s desire for independence was unquestionably sharpened.