Sullivan was an early and outspoken opponent of British colonial policies leading to the revolution. He was elected to the provincial assembly in 1774. When it first met in June, Sullivan was a leader in calling for a Continental Congress. When Governor Thomas Gage indefinitely delayed the next meeting of the assembly the following October, its members met anyway, establishing the Massachusetts Provincial Congress. This body exercised "de facto" control over Massachusetts during the early years of the American Revolutionary War. In addition to sitting in the provincial congress, Sullivan was a leading organizer of colonial defenses in York County, sitting on its committee of correspondence and other bodies. He was sent in 1775 as part of a commission to inspect the troops and facilities at Fort Ticonderoga in upstate New York, which was nominally under the control of Benedict Arnold, who had been issued a Massachusetts colonel's commission and succeeded in capturing it with the assistance of Ethan Allen. The arduous journey made Sullivan ill for several months afterward.