In the 1950s, Alvarado and Decoto – the latter now making up the eastern side of the town – were annexation targets of the nearby communities of Newark, Hayward, and what would become Fremont. On January 13, 1959, they decided to incorporate themselves into a single city, and named it after the Horners' original settlement, Union City. Tom Kitayama served as the city's first mayor in 1959 and was involved in Union City politics for 32 years until his retirement in 1991. The population of the city grew from 6,000 in 1959 to 74,000 in 2019.In 2016, Union City started to rebrand and got a new city seal. Union City opened a new Teen Center in 2018.