In 1772, he moved to Elizabethtown in the colonial-era Province of New Jersey, where he rented a house in town. A young Alexander Hamilton lived with Livingston for at least the winter while he attended Francis Barber's grammar school.

Livingston started construction of a large country home to house his growing family. The house, known as Liberty Hall, still stands. After attaining considerable influence among the local patriots, Livingston was elected to serve as one of New Jersey's delegates to the Continental Congress in Philadelphia, where he served from July 1774 to June 1776. New Jersey's Provincial Congress declined to reappoint him to the Second Continental Congress, however, since he did not favor immediate independence, so he was not a signatory to the Declaration of Independence that was unanimously adopted on July 4, 1776. William Livingston's older brother, Philip Livingston, who remained as a strong member of the New York delegation, did become one of the only 56 signers.

Meanwhile, the New Jersey Provincial Congress, instead, offered William Livingston, command of its state's militia, an offer he declined. So when William Livingston returned to New Jersey from Philadelphia that summer of 1776, he relied on his previous commission (of October 1775) as a brigadier general of the New Jersey Militia.