Warner, an African-American, was born on Hookstown Road in Baltimore County, Maryland to a father who was a farmer and ex-slave who acquired his freedom one year before Warner was born. Warner's date of birth is unclear. Some records show that he was born on April 19, 1815. However, American Colonization Society documents list him as age nine when he emigrated to Liberia, with eight relatives, on the ship Oswego in 1823.That would put his birth year as 1814.
A member of the Americo-Liberian elite, he also served as a member of the Liberian House of Representatives and Liberian Senate.In 1877, he became an agent of the American Colonization Society.
Warner's main concern as President was how the indigenous people, particularly the indigenous people in the interior, could be brought into society and become cooperating citizens. He organized the first expedition into the dense forest, led by Benjamin J. K. Anderson. In 1868, Anderson journeyed into Liberia's interior to sign a treaty with the king of Musardo. He took careful notes describing the peoples, the customs, and the natural resources of those areas he passed through, writing a published report of his journey. Using the information from Anderson's report, the Liberian government moved to assert limited control over the inland region.