Mary Ellen Keller was born on May 6, 1858, in Minnesota to Samuel J. Keller and Melvina Wolleat. Her father was a brick maker who owned $1,200 of real estate and $200 of personal property in 1860. The family moved to Bluffton, Indiana, around 1858, and she grew up and attended school there. She married Frederick Mosiman on February 8, 1882, and they had three children: Ralph, born around 1883; Hugh, born around 1886; and Howard, born around 1901.
By 1910, the family was living in Elwood, Indiana, and her husband was working as a merchant. Her husband died on February 22, 1915, and she returned to Bluffton sometime before 1930. She died of arteriosclerosis in Indianapolis, Indiana, on October 1, 1934.