Present day Dearborn County is an eclectic mix of historic buildings and new companies. The county was founded shortly after the formation of the Indiana Territory in 1800, and is the third oldest county in Indiana. Dearborn was organized in 1803 by Governor William Henry Harrison and was named after General Henry Dearborn, President Thomas Jefferson's secretary of war.

The present boundaries of Dearborn County were established in 1845: Franklin County forms the northern boundary; Hamilton County, Ohio, forms the eastern boundary; Ohio County and Laughery Creek form the southern boundary; and Ripley County is the western boundary.

The location of the county has made trade a focal point of its economy. Trade in the county began to flourish with the construction of the Whitewater Canal in the 1836 to 1843 period and the arrival of the Ohio and Mississippi Railroad in the 1850s. The Civil War however slowed the progress of county commerce when John Morgan Hunt's soldiers swept through much of southeastern Indiana, destroying many railroad bridges.

The county's economic growth experienced a revival in the post-war period with the establishment of several manufacturing venues. Companies established in the second half of the nineteenth century include:

• Joseph E. Seagrams and Sons (1857)
• Aurora Casket Company (1890)
• A.D Cook Pump Company (1870)
• Cochran Chair Company (1879)

These companies established the county as a manufacturing stronghold, a trend that has continued throughout the twentieth century. In recent years, the county's economy has begun the process of diversification with the arrival of such companies as the Argosy Casino and Hotel in 1997.