Francis Marion Selsor was born around 1836 in Indiana to Andrew and Nancy Selsor. His father was a farmer who owned $1,500 of real estate and $600 of personal property. The family lived in Brown, Indiana, until the 1850s, when they moved to Medicine, Missouri. He married a woman named Helen in the 1850s, and they had at least three children: Mary, born around 1858; Henry, born around 1861; and Sherman, born around 1866. Selsor worked as a farmer in Medicine, and by 1860, they owned $1,200 of real estate and $450 of personal property.
He enlisted in the Union army on September 1, 1861, and he mustered in as a private in Company C of the 23rd Missouri Infantry later that day. The regiment took part in the Battle of Shiloh and the Atlanta campaign. He mustered out on September 22, 1864.
Selsor returned to Medicine after the war and resumed his work as a farmer. He applied for a federal pension in November 1888, but he died before he could receive one. He died on April 21, 1891.