The town is in the south-central part of the county, west of Binghamton. The communities of Union form the western suburbs of Binghamton. Two communities, the villages of Johnson City and Endicott, along with Binghamton, make up the "Triple Cities."
The region was first settled by Americans/Europeans around 1782.
During the American Revolution, detachments of soldiers were sent through this region with the intent of attacking natives who were friendly to the British. Documented attacks on native villages in present-day Binghamton, Vestal, and Choconut Township, Pennsylvania, indicate that many natives were killed in these incursions in 1779. Of particular note are the advances of General Poor and General Sullivan, each leading troops on separate fronts. Their soldiers reunited at the site of Union, hence the name.
The town of Union was established in 1791, while in Tioga County before the creation of Broome County. The town, a "mother town" of the county, was later reduced in size by the formation of other towns, the first being the town of Owego (then called the "Town of Tioga") in 1800, followed by Lisle (1801), Greene (1808, now in Chenango County), Vestal (1823), and Maine (1848).