The territory in which Dormont Borough is situated was held by the Delaware and Shawnee tribes until 1768 when the territory was part of the transaction in which Fort Stanwix was purchased from the Six Nations. In following years, Dormont’s area was part of Cumberland County, Pitt Township in Bedford County, Penn Township in Washington County, and finally, St. Clair Township in Allegheny County in 1788.

Dormont was originally fashioned as a borough from parts of Scott and Union Townships. An order of incorporation was signed on March 31, 1909, making Dormont the first independent municipality in the South Hills of Allegheny County. The first election of Borough officials was held on April 27, 1909. Dormont later annexed part of Scott Township in 1909, two additional parts of Union Township in 1913 and 1916, and a portion of Mt. Lebanon Township in January of 1921, giving it a total area of just less than one square mile.

Municipal officials initially wanted to name the new community "Mt. Lebanon;" however, objections were raised by their as yet unincorporated neighbors to the south, who eventually adopted the name. Instead, the name Dormont was chosen from the Americanized version of the French term "mont d'or," loosely translated as "Mountain of Gold."

The first election of borough officials was held on April 27, 1909. Dormont later annexed part of Scott Township in 1909, two additional parts of Union Township in 1913 and 1916, and a portion of Mt. Lebanon Township in January 1921, giving it a total area of just less than one square mile. With the construction of the streetcar tunnel and the Liberty Tunnels came rapid growth.

Once the borough of Dormont was incorporated in 1909, at least 30 different streets were given new names. Today, all but two of Dormont's "side streets" are designated as avenues, the exceptions being Memorial Drive and Park Blvd. Two of Dormont's mainline streets, Scott Road and McFarland Road, are two-lane "borough line" streets in which one lane is located in Dormont and the other in Mt. Lebanon. The northern stretch of another "borough line" street, McNeilly Avenue, runs through Dormont on one side and the Pittsburgh neighborhood of Brookline on the other. Before the incorporation and street name changes, Dormont actually had four designated "streets": Beech St. (Dwight Ave.), Sycamore St. (Philadelphia Ave.), Sylvester St. (Texas Ave.) and "C" St. (Miles Ave.).

The population was 8,244 at the 2020 census.