When Seth Millington Blair Jr. was born on 19 March 1844, in DeWitt, Texas, United States, his father, Seth Millington Blair, was 25 and his mother, Cornelia Jane Espy, was 24. He lived in Salt Lake, Utah Territory, United States in 1850 and Salt Lake, Utah, United States in 1860. He registered for military service in 1853. He died on 4 February 1867, in Salt Lake City, Salt Lake, Utah Territory, United States, at the age of 22, and was buried in Salt Lake City Cemetery, Salt Lake City, Salt Lake, Utah Territory, United States.