The selection of the site for the town was the result of an act of the Missouri Legislature in 1825. A majority of taxpayers of Franklin County signed petitions calling for the county seat to be moved from Newport, on the Missouri River, to a location near the center of the county. The legislative act appointed three men to select the most suitable location for a county seat within 3 miles of the center of the county. Three commissioners were to purchase, or secure by donation, land which included the site selected for the county seat. The tract of land was to be not less than 50 nor more than 160 acres. According to the act, once the land was acquired, it was to be laid out in lots with streets. All lots were to be sold with the proceeds to be used for needed county buildings, building bridges and improving highways.

The land selected was donated by three farmers who lived in log cabins they had built on the land. In 1826, the 72 1/2 acres was laid out in 42 blocks and 7 fractional blocks.

The County Court began meeting in Union in June of 1827 in a double log house and continued to meet there until the new Courthouse was completed in 1828. Tradition says the new building was of logs, but according to the deed it was brick. In 1849, the building was replaced by a new two-story brick structure and, in 1923, construction of the current Courthouse was completed. All three have been located on the site of the original public square.

Business and houses began going up immediately. Added to the town in its first 15 years were a blacksmith, two general stores, a wagonmaker, a cabinet maker, a shoemaker, a hotel, a tavern, and a four-story flour mill. The post office was opened on July 20, 1827.

Union continued to grow at a moderate rate until 1887 when it experienced a "boom" caused by the completion of the St. Louis, Kansas City and Colorado Railroad to this point. According to newspaper accounts published at the time, there was much excitement and a huge celebration when the track finally reached town. A giant arch of evergreen was placed over the track of Washington Avenue, a band played and refreshments were served. A roundhouse and machine shops of the railroad were opened here. In the four months following this event, forty houses were added to the town and the population grew to 610.

While most of the town's earliest settlers were homesteaders who came from Virginia and other states east of the Mississippi, settlers from Germany started arriving in the 1840's. Some give the Germans the credit for improving the town and adding beauty by planting trees and laying out gardens. In an article in a St. Louis paper in 1895, the writer stated "So, today, Union is a wilderness of shade and greenery, dotted profusely with flower gardens, whose brilliant colors are visible on every hand. The people are hospitable to a great degree." This describes the Union of today as well.

Union has a population of 5,909 (1990 census). The Courthouse still dominates the central business district of Union and much of the activity is related to county government. A wide variety of stores and services are available to area residents. Manufacturing is important to the city's economy and there is room for expansion in the city's four industrial sites. The local school system is AAA rated and the city also has parochial and private schools. Because of Union's central location, East Central College and the headquarters of the regional library system are here. There are approximately 18 churches, and many civic services and cultural organizations.