He later became US consul to Bremen in 1842 and was appointed to negotiate commercial treaties with Hanover, Oldenburg, and Mecklenburg in 1845 as well as all the German states except Prussia in 1847.

During the American Civil War, he sided with the Confederacy and devoted himself especially to the development of the material interests of its states.

Yancey and Rost were later replaced by John Slidell and James Murray Mason, the two subjects of the Trent Affair.

Mann spent the latter part of his life living in France where he had an apartment in Paris and a country house in Chantilly.

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.