Monessen experienced rapid growth in the first two decades of the twentieth century, the population increasing from 2,197 in 1900 to 11,775 in 1910 and then to 18,179 in 1920.
While there were many companies operating in Monessen, the largest employer was Pittsburgh Steel Company, later renamed Wheeling-Pittsburgh Steel. In a practice that is shocking by today’s standards (and not limited to Monessen), pay was determined by ethnic background. For example, a Welsh immigrant would be paid more than an Italian immigrant. A normal workweek was 84 hours (7 days times 12 hours). Employers did not adopt the eight hour workday until the 1920s.
Monessen’s status changed from “borough” to “city” on September 16, 1921.
In the years following World War II, the region’s manufacturers, especially the steel industry, found it harder to compete, leading to a period of employment cutbacks. In 1972, the closure of Page Steel and Wire Company was a major setback to the city. A far greater blow to Monessen occurred when its largest employer, Wheeling-Pittsburgh Steel, closed nearly all its Monessen operations in 1986. (The company’s rail mill did not close until March 1987, due to a bad batch of rails that were shipped.) The mill’s closure marked the end of an era in Monessen's history.
According to the United States Census Bureau, the city has a total area of 3.1 square miles (7.9 km²), of which, 2.9 square miles (7.5 km²) of it is land and 0.2 square miles (0.4 km²) of it (4.90%) is water.
There were 3,916 households out of which 21.2% had children under the age of 18 living with them, 42.9% were married couples living together, 15.2% had a female householder with no husband present, and 37.4% were non-families. 34.3% 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 2.19 and the average family size was 2.80.
In the city the population was spread out with 19.6% under the age of 18, 5.3% from 18 to 24, 23.3% from 25 to 44, 22.7% from 45 to 64, and 29.1% who were 65 years of age or older. The median age was 46 years. For every 100 females there were 84.8 males. For every 100 females age 18 and over, there were 79.0 males.
The median income for a household in the city was $26,686, and the median income for a family was $37,269. Males had a median income of $34,773 versus $21,508 for females. The per capita income for the city was $16,627. About 11.5% of families and 15.7% of the population were below the poverty line, including 27.7% of those under age 18 and 8.2% of those age 65 or over.