On returning to civilian life, he became a successful society photographer in Saint Louis, Missouri. At 13, he set out on his own and worked for the Merrill Drug Company in Saint Louis and Western Union. He joined the Union Army as a teenager and served under Generals William Tecumseh Sherman, Nathaniel Lyon and Ulysses South. Grant.
Medal of Honor citation After the war, he returned to Saint Louis and did menial jobs at a photographic gallery.
He found better pay stringing telegraph wire for a railroad, but returned to photography, going into partnership and setting up Remington, Guerin, and Mills Gallery in Ottumwa, Iowa. He was eventually bought out and returned to Saint Louis, where he worked for several established photographers, learning the trade.
Finally, in 1876, he set up shop on his own. He established a reputation, received international recognition for his portraits, and was several times president of the National Photographic Society.
He opened several more galleries in the city, owning a total of six over his 27-year career.
Pioneering women photographers Emme and Mayme Gerhard studied with him for three years. When he retired in January 1903, he sold his studio to them. Guerin died of a heart attack on July 11, 1903.
A 1982 American Heritage magazine article labeled him a "turbid Victorian hack", though it did concede he was technically gifted.
Some of his photographs are held by the Library of Congress. Photographs by Guerin.