Winthrop is located at . According to the United States Census Bureau, the city has a total area of, all land. Winthrop is located north of U.S. Route 20.
As of the census of 2010, there were 850 people, 346 households, and 228 families living in the city. The population density was. There were 357 housing units at an average density of. The racial makeup of the city was 98.7% White, 0.1% Native American, 0.2% Asian, 0.1% from other races, and 0.8% from two or more races. Hispanic or Latino of any race were 0.4% of the population. There were 346 households of which 31.8% had children under the age of 18 living with them, 52.3% were married couplesliving together, 9.0% had a female householder with no husband present, 4.6% had a male householder with no wife present, and 34.1% were non-families. 28.9% of all households were made up of individuals and 14.5% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 3.00. The median age in the city was 38.5 years. 27.5% of residents were under the age of 18; 5.7% were between the ages of 18 and 24; 24.2% were from 25 to 44; 26.7% were from 45 to 64; and 16% were 65 years of age or older. The gender makeup of the city was 50.0% male and 50.0% female.
2000 census
As of the census of 2000, there were 772 people, 327 households, and 206 families living in the city. The population density was 1,330.9 people per square mile. There were 341 housing units at an average density of 587.9 per square mile. The racial makeup of the city was 99.09% White, 0.39% Native American, 0.26% from other races, and 0.26% from two or more races. Hispanic or Latino of any race were 0.39% of the population. There were 327 households out of which 30.6% had children under the age of 18 living with them, 52.0% were married couples living together, 8.3% had a female householder with no husband present, and 37.0% were non-families. 33.9% of all households were made up of individuals and 17.7% had someone living alone who was 65 years of age or older. The average household size was 2.35 and the average family size was 3.01. 25.5% were under the age of 18, 8.7% from 18 to 24, 27.5% from 25 to 44, 22.2% from 45 to 64, and 16.2% were 65 years of age or older. The median age was 38 years. For every 100 females, there were 91.1 males. For every 100 females age 18 and over, there were 84.3 males. The median income for a household in the city was $36,136, and the median income for a family was $42,969. Males had a median income of $31,641 versus $22,500 for females. The per capita income for the city was $19,183. About 4.3% of families and 5.7% of the population were below the poverty line, including 6.3% of those under age 18 and 7.8% of those age 65 or over.
