The origin of the name of the river is unknown; among several explanations is the theory that it comes from the Pottawatomie word Sain-guee-mon (pronounced "sang gä mun"), meaning "where there is plenty to eat.

Prior to being elected President of the United States, Abraham Lincoln represented Sangamon County in the Illinois Legislature.

Lincoln, along with several other legislators, was instrumental in securing Springfield, the Sangamon County seat, as the state's capital.

The careers of these men and the people with whom they collaborated helped to shape much of the history and development of Sangamon County, Illinois.

Despite its relatively large population and the presence of Springfield, the state's capital and a significant urban center, the county has remained reliably conservative.

In 2024, sheriff deputy Sean Grayson was fired after killing Sonya Massey, an unarmed woman, within her home located in Woodside Township near Springfield, Illinois following her call for assistance.

[17] Massey's father, community members, and elected officials raised strong concerns about the department, after it was revealed prior to Grayson's hiring in 2023.

[18] Sheriff Jack Campbell who hired Grayson, after receiving calls to resign, including from Governor J.B Pritzker announced he would step down effective August 31, 2024.