In 1632, Samuel de Champlain included this area in the region belonging to the Illinois, and in the 18th century the Sauk mined lead within the present county limits.

By the time the first white settlers arrived there, all of Green County was the property of the Ho-Chunk, who referred to the mines as the "Sac Diggings."

The federal government recognized Indian title to frontier land, and generally forbade the private sale of Indian land to individuals, but as squatters continued to work the mines in southwestern Wisconsin, conflict arose between them and the Ho-Chunk, as well as the Sauk and Meskwaki, all of whom mined and sold lead.

In 1832, Black Hawk attempted to bring his people back into their former Illinois lands, they were pursued across the border into Wisconsin, then known as western Michigan Territory.

The Ho-Chunk were divided in the Black Hawk War, with many warriors siding with the Sauk, and others taking personal revenge on frontier settlers in the lead mining district.

Although many other Ho-Chunk aided the US army in pursuing the Sauk, they were coerced into selling their lands south of the Wisconsin River in a treaty signed in September, one month after Black Hawk's surrender.

Prior to that year, the only times they failed to win the county were in the midst of a divided party vote in 1912, the presence of Wisconsinite Robert La Follette on the ballot in 1924, and national Democratic Party landslides in 1932, 1936, & 1964.

From 1992 onward, the county has backed the Democratic candidate in every presidential election, though their margins of victory have been often relatively narrow.