President Abraham Lincoln, United States Headquarters, Army of the Potomac, near Antietam, October 4, 1862. Lincoln is flanked by Major General John A. McClernand (on the right), and E. J. Allan, Chief of the Secret Service (on the left). The photograph was taken by Alexander Gardner (American, born Scotland, 1821 - 1882). Lincoln (1809-1865) was the 16th President of the United States, from March 1861 until his assassination in 1865. He led his country through the American Civil War, preserving the Union, ending slavery, and promoting economic and financial modernization.