Clark County, Arkansas was one of three counties in the future Arkansas Territory to be created by the legislature of Missouri Territory on Dec. 15, 1818.
It was named for the territorial governor, William Clark, the former explorer of the Lewis and Clark Expedition.
It comprised, roughly, all of the present Clark, Hot Spring, Garland, Pike and Dallas counties. The Arkansas Territory wasn't created until July 4, 1819.
Early county seats were at or near the homes of Jacob Barkman on the Caddo River and Adam Stroud near Hollywood, and the town of Biscoeville.
In 1880 the seat was established at Greenville. It remained there for 12 years, but was moved to Arkadelphia in 1892. 
The pastor and theologian L. L. Clover, founder of the Louisiana Missionary Baptist Institute and Seminary in Minden, Louisiana, was born in Clark County in 1902.
1. Clark County, http://www.clarkcountyarkansas.com/ 2. Encyclopedia of Arkansas History, http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?search=1&entryID=754