Frederick K. Fletcher was born around 1840 in Concord, New Hampshire, to Daniel and Margaret Fletcher. His father was a farmer who owned $2,000 of real estate and $1,100 of personal property by 1860. He grew up and attended school in Colebrook, New Hampshire.
He enlisted in the Union army on August 11, 1862, and he mustered in as a sergeant in Company H of the 13th New Hampshire Infantry on September 19. The regiment took part in the Battle of Fredericksburg. On February 4, 1864, Union officials promoted him to captain of Company F of the 19th USCT Infantry. The regiment took part in the siege of Petersburg and the Appomattox campaign. He supported President Abraham Lincoln in the election of 1864, and he expressed devotion to the Union. He declared the United States the “best and only free Government on earth.” He mustered out on January 15, 1867, and he died sometime afterward.