Robert B. Coffin was born around 1838 in Jamestown, North Carolina, to Abel and Maria Coffin. His father was a farmer who owned $800 of real estate and $450 of personal property by 1870. The family moved to Indiana around 1841, and he grew up and attended school there. He began work as a carpenter and miller around 1855, and he moved to Hennepin County, Minnesota, around 1860.
He enlisted in the Union army on August 13, 1862, and he mustered in as a private in Company D of the 6th Minnesota Infantry. The regiment took part in the Dakota War of 1862, and he mustered out on August 19, 1865.
He settled returned to Hennepin County after the war, and he earned a living as a farmer. He married Elizabeth Archibald on November 17, 1869, and they had at least two children: Orla, born around 1872; and William, born around 1874. By 1870, he owned $800 of real estate and $150 of personal property. They moved to Crow Wing County, Minnesota, around 1873.
He supported the Republican Party, and he reportedly “served in local affairs almost continuously.” He served as justice of the peace for sixteen years, and he also served as a district assessor. He applied for a federal pension in June 1890 and eventually received one. He died in Seattle, Washington, on August 24, 1918.