The area comprising the present Elm Street section of Ansonia and Derby Avenue section of Derby was first settled in 1652 and was originally a part of the township of Derby. In 1844, merchant and philanthropist Anson Greene Phelps (1781–1853) wanted to expand the old Borough of Birmingham (the present down town of the City of Derby) up along the west side of the Naugatuck River. Unfortunately since he was not able to purchase the land required from its owner, Anson Phelps acquired land along the east side of the river, which today is the city's downtown section, in 1844. A canal was dug to power the factories and businesses in the new industrial village and named it Ansonia. The name came about when Mr. Phelps wanted to call his new industrial village "Phelpsville" but found out there was another village in the region by that name. Under the suggestion of a friend, Mr. Phelps Latinized his first name thus Ansonia came about. Soon Ansonia became the most populous area of Derby and boasted many factories. The state chartered Ansonia as a borough of Derby in 1864 and amended it once again in 1871 granting full municipal privileges. In 1888, a petition was circulated in the borough of Ansonia for the purpose of becoming a separate township from Derby. In 1889 the State General Assembly granted the separation thus constituting the Borough, Hilltop, West Ansonia, and Elm Street areas as a separate town known as Ansonia. This was the 168th township in the State of Connecticut. In 1893, Ansonia was incorporated as a city, consolidating with the coterminous town and the old borough.
Derby Probate District covers Towns of Ansonia and Derby.