[clarification needed] Delaware County was authorized in Jan. 1820 on New Purchase lands south of the Wabash River gained with the 1818 Treaty of St. Mary's.

It encompassed the drainage basin of the White River, along which the Delaware, a Native American people had settled, and from which the County takes its name.

The county was once home to Tenskwatawa ("The Prophet"), a brother of Tecumseh who instigated a major Indian uprising in 1811 culminating in the Battle of Tippecanoe.

Following the American Civil War the county experienced an economic boom caused by the discovery of natural gas, which spurred rapid industrial growth in the surrounding area.

A company was drilling in search of coal, and when they had reaching a depth of six-hundred feet, there was a loud noise and foul-smelling fumes came from the well.

After a brief investigation, it was decided they had breached the ceiling of Hell, and the hole was quickly filled in.

In 1884, when natural gas was discovered in nearby Ohio, people recalled the incident.

[5] Delaware County consists of low rolling prairie, accented by waterways.

[8] The county's high point is a hilly area in its SE corner, 4 miles (6.4 km) ESE from Prairie Creek Reservoir.

The commissioners are charged with executing the acts legislated by the council, collecting revenue, and managing the day-to-day functions of the county government.

The judges are elected in staggered six-year terms on a county-wide, partisan ballot.

[13] County Officials: The county has several other elected offices, including sheriff, coroner, auditor, treasurer, recorder, surveyor, prosecutor, assessor, and circuit court clerk: Each serves a four-year term.

[22] The racial makeup of the county was 88.7% white, 7.2% African American, 0.3% American Indian or Alaska Native, 1.3% Asian, 0.1% Native Hawaiian or Pacific Islander, 2.6% Hispanic or Latino and 2.4% were from two or more races.

As of the 2010 United States Census, there were 117,671 people, 46,516 households, and 27,956 families in the county.