As governor, he prevailed in the U.S. Supreme Court case Ex parte Madrazzo, a dispute over whether a claim of ownership of a group of enslaved people could be enforced against the state.

[1] He served in the Georgia House of Representatives before he was elected to consecutive two-year terms as the 31st Governor, from 1819 to 1823.

During his term, he successfully defended states' rights in a US Supreme Court case, Ex parte Madrazzo, over a Spanish citizen who claimed that he owned some of Clark's slaves.

Old Town, in 1832 in what was then Washington County (now Bay County) and was buried in that same city; however, his grave was relocated to Marietta National Cemetery in Georgia in 1923 by the Georgia State Society Daughters of the American Revolution.

Clarkesville, Georgia[3] and Clarke County, Alabama are named after him.