[a] The land which became Sullivan County was originally purchased from the Iroquois by the Province of Pennsylvania in 1768, as part of the first Treaty of Fort Stanwix.

There are numerous river valleys in the southern and western parts of Sullivan County.

[7] Sullivan has a warm-summer humid continental climate (Dfb) and average monthly temperatures in Laporte range from 21.8 °F in January to 67.9 °F in July.

[8][failed verification] Sullivan County lies predominantly within the Appalachian Plateau physiographic province, which is characterized by gently folded and faulted sedimentary rocks of middle to late Paleozoic age.

The southern border of the county is approximately at the Allegheny Front, a geological boundary between the Ridge and Valley province and the plateau.

The stratigraphic record of sedimentary rocks within the county spans from the Devonian Lock Haven Formation (exposed only in Lick Creek valley) to the coal-bearing Pennsylvanian Allegheny Formation.

[9][10] Nearly all of Sullivan County was glaciated several times in the past, during the Pleistocene epoch, or "Ice Age."

Most of the county is covered by glacial till of Late Wisconsinan age.

Late Illinoian Stage deposits may underlie the Late Wisconsinan deposits, and these are exposed in the south central part of the county (roughly western Davidson Township).

Some streams along the eastern border of the county flow into the North Branch of the Susquehanna River.

Sullivan County is also home to a large, private hunting club, Painter Den, Inc.[27] This vast property is situated in Davidson, Laporte and Colley townships.

Painter Den Pond is also on the property and is stocked with perch and pike.

There are several festivities held in the county each year: Under Pennsylvania law, there are four types of incorporated municipalities: cities, boroughs, townships, and, in at most two cases, towns.