New Lenox Township is located in the heart of Will County, Illinois, on U.S. Route 30 and Interstate 80. As of the 2010 census, its population was 40,270, and it contained 13,721 housing units. New Lenox Township has thirty-six square miles within its jurisdiction. It serves almost all of the Village of New Lenox and parts of the individual municipalities of Mokena, Homer Glen, Joliet and the entire unincorporated areas of the New Lenox community. New Lenox Township changed its name from Vernon Township on June 10, 1850.
New Lenox Township provides the fair and equal assessment of all real estate in the Township, and road, bridge and storm water system maintenance outside the municipalities. New Lenox Township also provides environmental programs, mosquito abatement, transit services, senior housing, family, senior and youth service programs, a food pantry for New Lenox Township residents in need, general assistance to needy families in New Lenox Township, and the administration, maintenance and preservation of the local Marshall and Maplewood Cemeteries.
According to the 2010 census, the township has a total area of , of which (or 99.78%) is land and (or 0.22%) is water.
New Lenox was also known as Spencer during the early 1900s.