Frederick Mosiman was born around 1858 in Indiana to Jacob Mosiman. His father was a German immigrant who worked as a farmer, and by 1860, he owned $800 of real estate and $100 of personal property. The family lived in Vera Cruz, Indiana, in 1860, but they moved to Bluffton, Indiana, sometime in the 1860s. Mosiman grew up and attended school in Bluffton, and by 1880, he was working as a clerk in a store.
He married Mary Ellen Keller on February 8, 1882, and they had three children: Ralph, born around 1883; Hugh, born around 1886; and Howard, born around 1901. They settled in Pipe Creek, Indiana, and Mosiman worked as a merchant in the shoe industry. They moved to Elwood, Indiana, sometime before 1910, and he died of apoplexy there on February 22, 1915.