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is a city that straddles the border of Franklin County and Crawford County in the U.S. state of Missouri. The population was 6,351 at the 2000 census. Stephen Sullivan founded the city in the late 1850s and cleared brush, which facilitated the building of the railway depot there, and the railroad named the town and station after him in 1860. In 1856, when a post office was established in present-day Sullivan, the local postmaster named the place "Mount Helicon". This short-lived name was after an actual mountain in Greece that was the mythical sanctuary of the Muses. Between 1920 to 1960 the city grew from 900 to more than 4,000 residents, making Sullivan the second-fastest growing city in the state during that time.
According to the United States Census Bureau, the city has a total area of 7.7 square miles (19.9 km²), all land.