Formed 7 March 1803 from Clark County, Indiana Territory. Area was part of Clark County for a short time between Ohio statehood and formation of Dearborn County. Previously it had been a part of Hamilton County, Northwest Territory.
- 23 November 1810 – Lost and gained land in formation of Jefferson, Franklin, and Wayne counties, Indiana Territory
- 7 September 1814 – Lost Land in formation of Switzerland County
- 27 December 1816 – Lost land in formation of Ripley County
- 4 January 1844 – Lost land in formation of Ohio County
- 7 January 1845 – Lost additional land to Ohio County. Present boundaries formed.
- History of Dearborn and Ohio Counties, Indiana, Weakley & Company, 1885.
- Pence, George and Nellie C. Armstrong. Indiana Boundaries: Territory, State and County. 1933. Reprint, Indianapolis: Indiana Historical Bureau, 1967.
- Shaw, Archibald, ed., History of Dearborn County, Indiana…Bowen, 1915.
- Works Progress Administration of Indiana.Dearborn County, Indiana, Index of Names of Persons and Firms. Indianapolis: Indiana State Library, 1938.