Marshfield is in Wood and Marathon counties in the state of Wisconsin. It is located at the intersection of U.S. Highway 10, Highway 13 and Highway 97. The largest city in Wood County, its population was 18,929 at the 2020 census. Of this, 18,119 were in Wood County, and 810 were in Marathon County. The city is part of the United States Census Bureau's Marshfield-Wisconsin Rapids Micropolitan Statistical Area, which includes all of Wood County (2020 population: 74,207). The portion of the city in Marathon County is part of the Wausau Metropolitan Statistical Area.

Marshfield is home to the Marshfield Clinic, a large healthcare system that serves much of Central, Northern, and Western Wisconsin. In 2010, Marshfield was ranked 5th in a list of "The Best Small Cities to Raise a Family" compiled by Forbes magazine.