Flat prairie lie north of Quincy and, as the Mississippi turns sharply to the southwest bluffs, floodplain, and farmland lie to the south and southwest of the city. East of Quincy lies the Quincy Hills geographic region, a region of hills that are similar to the Lincoln Hills in Missouri, where a mixture of rolling hills, woods, and farming shape the landscape. Adjacent to Quincy, on the Missouri side of the river, is West Quincy, Missouri and a vast floodplain used primarily for farming. The city is situated on bluffs overlooking the Mississippi River. According to the 2010 census, Quincy has a total area of 15.946 square miles. Quincy is adjacent to the Mississippi River and Quincy Bay, a large inlet of water fed by Cedar and Homan Creeks. The city has several historic districts, including the Downtown Quincy Historic District and the South Side German Historic District, which display the architecture of Quincy's many German immigrants from the late 19th century. It was Illinois' second-largest city, surpassing Peoria in 1870. During the 19th century, Quincy was a thriving transportation center as riverboats and rail service linked the city to many destinations west and along the river. As of July 1, 2015, the Quincy Micro Area had an estimated population of 77,220.

The 2020 census counted a population of 39,463 in the city itself, down from 40,633 in 2010. Quincy, known as Illinois's 'Gem City', is a city in and the county seat of Adams County, Illinois, United States, located on the Mississippi River.

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