Warren was initially inhabited by Native Americans of the Seneca nation. French explorers had longstanding claims to the area which they acted to secure in an unambiguous fashion with a military-Amerindian expedition in 1749 that buried a succession of plaques claiming the territory as France's in response to the formation of the colonial Ohio Companyand the first of these was buried in Warren but ultimately control was transferred to the British after the French and Indian War. After the Revolutionary War, General William Irvine and Andrew Ellicott were sent to the area to lay out a town in 1795. It was named after Major General Joseph Warren. The first permanent structure in Warren, a storehouse built by the Holland Land Company, was completed in 1796. Daniel McQuay of Ireland was the first permanent inhabitant of European descent. Lumber was the main industry from 1810-1840, as the abundance of wood and access to water made it profitable to float lumber down the Allegheny River to Pittsburgh. David Beaty discovered oil in Warren in 1875 while drilling for natural gas in his wife's flower garden. Oil came to dominate the city's economy. Many of the town's large Victorian homes were built with revenue generated by the local oil and timber industries. In recent years, Warren has struggled through hard economic times and a steady decline in population, but the city is attempting to bounce back with the Impact Warren project, a riverfront development project in downtown Warren. The completed project will include new townhouses and senior citizen housing, retail and commercial development, a parking garage, convention center and bus depot. Major employers include Walmart, the United Refining Company, Allegheny National Forest, Northwest Savings Bank, Whirley-Drinkworks, , , , Blair Corporation, Sheetz, and . The Warren Historic District, A.J. Hazeltine House, John P. Jefferson House, Struthers Library Building, Warren Armory, Warren County Courthouse, Wetmore House, Guy Irvine House and Woman's Club of Warren are listed on the National Register of Historic Places.
As of the census of 2000, there were 10,259 people, 4,565 households, and 2,606 families residing in the city. The population density was 3,508.3 people per square mile. There were 5,046 housing units at an average density of 1,725.6 per square mile. The racial makeup of the city was 98.53% White, 0.20% African American, 0.20% Native American, 0.36% Asian, 0.02% Pacific Islander, 0.13% from other races, and 0.56% from two or more races. Hispanic or Latino of any race were 0.39% of the population. There were 4,566 households, out of which 27.2% had children under the age of 18 living with them, 43.0% were married couples living together, 10.8% had a female householder with no husband present, and 42.9% were non-families. 37.6% of all households were made up of individuals, and 16.4% had someone living alone who was 65 years of age or older. The average household size was 2.17 and the average family size was 2.87. In the city, the population was spread out, with 23.1% under the age of 18, 7.0% from 18 to 24, 28.4% from 25 to 44, 23.4% from 45 to 64, and 18.1% who were 65 years of age or older. The median age was 39 years. For every 100 females, there were 88.5 males. For every 100 females age 18 and over, there were 83.9 males. The median income for a household in the city was $32,384, and the median income for a family was $41,986. Males had a median income of $32,049 versus $22,969 for females. The per capita income for the city was $18,272. About 8.0% of families and 10.7% of the population were below the poverty line, including 11.5% of those under age 18 and 6.0% of those age 65 or over.
Gudrun Ensslin, founder member of German activist group the Red Army Faction, spent a year in the United States aged eighteen, where she attended high school in Warren. She graduated in the Honor Group at Warren High School in 1959.
Johnny Cash co-wrote and sang "As Long as the Grass Shall Grow", about the loss of Seneca nation land due to the construction of the Kinzua Dam 6 miles east of Warren, on his October, 1964 album.
Warren was one of the featured locations in History's Revelation: The End of Days. No actual filming took place in Warren.
