After a year apprenticing as a blacksmith in Ann Arbor, Michigan, Mann suffered an injury that forced him to abandon the study.

He sought a trade, but with the outbreak of the Civil War, Mann decided instead to raise a regiment for the Union Army.

Mann petitioned President Abraham Lincoln to deploy the regiment, and the unit finally saw action after the First Battle of Bull Run.

On January 2, 1862, the company met Major General Stonewall Jackson's Valley District Army (15,000 soldiers), but managed to retreat after a skirmish.

He served on the staff of General Alpheus S. Williams until leaving to join his regiment at the First Battle of Kernstown.

The unit then moved to South Carolina and the 39th took part in the Siege of Fort Wagner; Mann sent the announcement of its capture to General Quincy Adams Gillmore.

For his work in Norfolk, Mann was promoted to a full colonel (although he did not officially accept the position) and he was ordered to return to his command of the 39th.

However, after anarchy engulfed in the city in the following weeks, Major General Alfred Terry reassigned Mann to Norfolk with four regiments at his command.

He died in Oak Park, Illinois, on December 13, 1908, and was buried in Chicago in Rosehill Cemetery.