The Marcellus Formation is a vast geological layer of shale spanning Pennsylvania, West Virginia and parts of other states and Ontario, which is named for an outcropping in or near Marcellus.
Marcellus is at the east end of the Finger Lakes District and is at the north end of Otisco Lake. High hills, deep valleys, lake and streams, make the town remarkably scenic. Nine Mile Creek is a noted trout stream where New York State provides several access areas. Onondaga County's Marcellus Park is in the village of Marcellus and Baltimore Woods Nature Center is nearby.
The town was formed in 1794, but lost territory to new towns. The Town of Geddes was removed in 1798. The township of Camillus was taken off in 1799. Otisco was removed in 1806. In 1830, the west part of the town was used for the new Town of Skaneateles. Marcellus regained territory in 1840 from Spafford and Otisco.
As of the census of 2000, there were 6,319 people, 2,378 households, and 1,773 families residing in the town. The population density was 194.1 people per square mile (75.0/km²). There were 2,488 housing units at an average density of 76.4/sq mi (29.5/km²). The racial makeup of the town was 98.31% White, 0.41% Black or African American, 0.30% Native American, 0.22% Asian, 0.02% Pacific Islander, 0.06% from other races, and 0.68% from two or more races. Hispanic or Latino of any race were 0.84% of the population.
There were 2,378 households out of which 37.1% had children under the age of 18 living with them, 63.3% were married couples living together, 8.0% had a female householder with no husband present, and 25.4% were non-families. 22.3% of all households were made up of individuals and 9.5% had someone living alone who was 65 years of age or older. The average household size was 2.66 and the average family size was 3.14.
In the town the population was spread out with 28.2% under the age of 18, 6.0% from 18 to 24, 27.0% from 25 to 44, 25.6% from 45 to 64, and 13.2% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 95.3 males. For every 100 females age 18 and over, there were 92.7 males.
The median income for a household in the town was $51,881, and the median income for a family was $58,188. Males had a median income of $40,541 versus $32,234 for females. The per capita income for the town was $25,628. About 2.5% of families and 3.2% of the population were below the poverty line, including 3.9% of those under age 18 and 0.5% of those age 65 or over.