The southwest half of the township lies on what is known as the Hunterdon Plateau, the northwest corner consists of the Musconetcong Ridge and the northeast section is part of the lower-lying Newark Basin around Spruce Run Reservoir.

Clinton Town was formed on April 5, 1865, within portions of the township, and became an independent municipality in 1895.

[3] The name "Union" was chosen over the alternative "Rockhill", which was a community at the southern end of the township.

[23] Union Furnace and its forge produced cannonballs for the Revolutionary War and shoes for horses and oxen, as well as farm implements.

Farms and fields were established in areas where trees were felled to provide fuel for the furnace.

[1][2] Unincorporated communities, localities and place names located partially or completely within the township include Coles Mills, Grandin, Hensfoot, Jutland, Kingtown, Mechlings Corner, Mount Salem, Norton, Pattenburg, Perryville, Polktown and Van Syckel.

[27][28][29] Two large state facilities in the township, the Edna Mahan Correctional Facility for Women (with 650 inmates in 2018)[38][39] and Hunterdon Developmental Center (which had 480 residents as of 2017),[40] account for almost 20% of the residents counted by the Census Bureau.

[41] The 2010 United States census counted 5,908 people, 1,752 households, and 1,221 families in the township.

[42] As of the 2000 United States census[15] there were 6,160 people, 1,666 households, and 1,162 families residing in the township.

[4][45][46][47][48][49][50] In February 2019, the Township Committee selected Page Stiger from a list of three candidates nominated by the Republican municipal committee to fill the seat expiring in December 2019 that was vacated by Michael Sroka when he resigned from office the previous month.

[51] In 2010, the Township's Environmental Commission earned Union Township the distinction of becoming the first community in Hunterdon County to achieve certification under the Sustainable Jersey program, which works to help communities reduce waste, cut greenhouse gas emissions and improve environmental equity.

[54][55][56] For the 119th United States Congress, New Jersey's 7th congressional district is represented by Thomas Kean Jr. (R, Westfield).

[58] For the 2024-2025 session, the 23rd legislative district of the New Jersey Legislature is represented in the State Senate by Doug Steinhardt (R, Lopatcong Township) and in the General Assembly by John DiMaio (R, Hackettstown) and Erik Peterson (R, Franklin Township).

[65][66] Constitutional officers elected on a countywide basis are Clerk Mary H. Melfi (R; Flemington, 2026),[67][68] Sheriff Fredrick W. Brown (R; Alexandria Township, 2025)[69][70] and Surrogate Heidi Rohrbach (R; Lebanon Township, 2028).

[71][72] Two state facilities, the New Jersey Department of Corrections Edna Mahan Correctional Facility for Women (with 650 inmates in 2018),[38] and the New Jersey Department of Human Services Hunterdon Developmental Center (which had 480 residents as of 2017),[40] are located in Union Township.

[81] The Union Township School District serves students in pre-kindergarten through eighth grade.

[91] The school is part of the North Hunterdon-Voorhees Regional High School District, which also includes students from Califon, Glen Gardner, Hampton, High Bridge, Lebanon Township and Tewksbury Township, who attend Voorhees High School in Lebanon Township.