Bagnell was platted in 1882 when the railrad was extended through the area. The city was named for William Bagnell of St. Louis County, who operated a tie business along the Osage River. A post office called Bagnell was established in 1884, and remained in operation until 1942.

Bagnell is located at 38°13′45″N 92°36′21″W (38.229284, -92.605967). According to the United States Census Bureau, the city has a total area of 0.60 square miles (1.55 km2), of which, 0.47 square miles (1.22 km2) is land and 0.13 square miles (0.34 km2) is water.

As of the census of 2010, there were 93 people, 43 households, and 24 families residing in the city. The population density was 197.9 inhabitants per square mile (76.4/km2). There were 60 housing units at an average density of 127.7 per square mile (49.3/km2). The racial makeup of the city was 94.6% White and 5.4% from two or more races. There were 43 households of which 32.6% had children under the age of 18 living with them, 34.9% were married couples living together, 16.3% had a female householder with no husband present, 4.7% had a male householder with no wife present, and 44.2% were non-families. 32.6% of all households were made up of individuals and 4.6% had someone living alone who was 65 years of age or older. The average household size was 2.16 and the average family size was 2.71. The median age in the city was 42.3 years. 23.7% of residents were under the age of 18; 8.6% were between the ages of 18 and 24; 27% were from 25 to 44; 34.4% were from 45 to 64; and 6.5% were 65 years of age or older. The gender makeup of the city was 49.5% male and 50.5% female.

As of the census of 2000, there were 86 people, 36 households, and 20 families residing in the town. The population density was 183.3 people per square mile (70.6/km²). There were 37 housing units at an average density of 78.9 per square mile (30.4/km²). The racial makeup of the town was 96.51% White and 3.49% Native American. Hispanic or Latino of any race were 1.16% of the population. There were 36 households out of which 30.6% had children under the age of 18 living with them, 38.9% were married couples living together, 11.1% had a female householder with no husband present, and 41.7% were non-families. 38.9% of all households were made up of individuals and 22.2% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 3.00. In the town the population was spread out with 27.9% under the age of 18, 9.3% from 18 to 24, 29.1% from 25 to 44, 20.9% from 45 to 64, and 12.8% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 95.5 males. For every 100 females age 18 and over, there were 100.0 males. The median income for a household in the town was $25,313, and the median income for a family was $27,857. Males had a median income of $22,083 versus $16,563 for females. The per capita income for the town was $14,633. There were no families and 2.2% of the population living below the poverty line, including no under eighteens and 11.1% of those over 64.