John Calvin Sloss was born in April 1840 in Harrison County Ohio. Although originally trained as a teacher, he came to Montana around 1870 to work in the mines. Early on he established some claims in the Bryants Mining Distrcit, the Elkhorn Lode and the True Fissure. After he had established himself in the mining field, he began to brnach out in the devlopment of other mines. It is believed he may have helped develop the Elhorn Skyline Mine. He became an expert, most likely in hydraulic mining. Searving as a consultant for investors, he visited mines to write reports to assist investors in their financial decisions . He built a strong relationship with two investors in particular, Achille F. Migeon (an industrialist from Connecticut) and Philip N. Moore (a mining engineer later based in St. Louis). He appeared to have been pretty well known in the mining buisness, in Montana and the west in general. He eventually moved with his family to the Capital Hill district in Seattle, Washington where he passed away in 1917.

In 1882 John Sloss maried Isabel Shott. They has three children: Martha E. Sloss, Isabel Sloss, and John Cameron Sloss Jr. Years following John Sr.'s death, John Jr. sold and gave family members stock in the company involved in developing the above mentioned Elkhorn Skyline Mine.