[2] The county was named for Charles M. McClain, an Oklahoma constitutional convention attendee.

[3] McClain County is part of the Oklahoma City, OK metropolitan statistical area.

The Chickasaw tribe began moving into this area in 1837, when the land had already been assigned to the Choctaws by the U.S. government.

The U.S. Army built Camp Arbuckle in 1850 to protect the road, but the troops were withdrawn to what is now Garvin County, Oklahoma in the following year.

A group of Delaware Indians occupied the former camp, then known as Beaversville, but left before the outbreak of the Civil War.

He and Jesse Chisholm, who acted as the negotiator, obtained an agreement with the Chickasaw leaders to allow ranching on their land, provided no whites were employed.

In 1906 the Oklahoma Central Railway (sold to AT&SF in 1914) built a line that traversed McClain County from the southeast to the northwest.

[3] As of the 2010 United States census, there were 34,506 people, 12,891 households, and 9,785 families residing in the county.

Mid-America Area Vo-Tech opened in 1971 to provide vocational education to students.

SH-9 enters McClain County via the McCall bridge, duplexed with I-35, and immediately turns due west, running along the northern edge of Goldsby until its intersection with U.S. Highway 62/U.S.

From this intersection the three highways continue southwest towards Blanchard and further on to Chickasha in Grady County.

(e.g. Pennsylvania Avenue would, if extended northward, eventually connect with the street of the same name in Oklahoma City.)

In 2010, the EPA ordered local water utilities to begin the first nationwide tests for hexavalent chromium 6 (AKA The Erin Brockovich Chemical).

[15] The Purcell water supply tested positive for an average of 11.53ppb, 577 times the original recommendation from the scientists at the respected and influential California Office of Environmental Health Hazard Assessment of 0.02ppb.

[16] The following sites in McClain County are listed on the National Register of Historic Places: