Union City is a city in Alameda County, California, United States. It was incorporated in 1959, combining the communities of Alvarado and Decoto. Alvarado was the original county seat of Alameda County, and the site of the first county courthouse is a California Historical Landmark. The city celebrated its 50th Anniversary in 2009. The population was 69,516 as of the 2010 U.S. Census.