Born in Mercer County, Kentucky, Harlan attended school before working as a clerk in a dry goods store from 1817 to 1821.

Deciding to embark upon a legal career, he read law under the guidance of a local judge before gaining admission to the bar in 1823.

Harlan commenced practice in Harrodsburg, Kentucky and enjoyed a busy but not especially-remunerative legal career.

He served until 1859 as the state's attorney general (during which time he wrote The Code of Practice in Civil and Criminal Cases).

Two years later, Harlan was appointed U.S. Attorney for Kentucky by President Abraham Lincoln, and he served in that capacity until his death in Frankfort on February 18, 1863.