Mann was born on April 26, 1801 in Hanover Courthouse, Virginia. He studied at the United States Military Academy, but left before he graduated. He later became American consul to Bremen in 1842 and was appointed to negotiate commercial treaties with Hanover, Oldenburg, and Mecklenburg in 1845 as well as all of the German states except Prussia in 1847. In 1849 he became commissioner to Hungary and in 1850 became U.S. Minister to Switzerland where he negotiated a reciprocity treaty. He came back to the United States afterwards and was appointed the first ever United States Assistant Secretary of State in 1853 which he served as until 1855.
During the Civil War, he sided with the Confederacy and devoted himself especially to the development of the material interests of the southern states. On March 16, 1861, Confederate President Jefferson Davis and Secretary of State Robert Toombs appointed Mann, William Lowndes Yancey and Pierre Adolphe Rost the first Confederate commissioners to Europe. The three sailed on March 31, 1861. Mann eventually received the title Commissioner of the Confederate States of America for Belgium and the Vatican. Yancey and Rost were later replaced by John Slidell and James Murray Mason.
Mann spent the latter part of his life living in France where he had an apartment in Paris and a country house in Chantilly. He wrote his memoirs which were available to read by 1888. Mann died in France in 1889, the exact date of his death (Nov. 15, 1889) being announced in the Nov. 16, 1889 issue of the French newspaper Journal des Debats. After a delay of about 6 weeks, Mann was finally interred on Jan. 2, 1890 in the Cimetiere du Montparnasse in Paris.
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|- style="text-align: center;"
|width="30%" align="center" rowspan="1"|Preceded by
(none) |width="40%" style="text-align: center;" rowspan="1"|United States Assistant Secretary of State
March 23, 1853 – May 8, 1855 |width="30%" align="center" rowspan="1"| Succeeded by
William Hunter |- |}