After two years at the saltworks, in 1872 Mantle arranged to learn telegraphy from an experienced operator, who agreed to train him on the condition that Mantle maintain the Western Union line along the Central Pacific tracks between Green River, Utah and Ogden, Utah during the winter months. In 1873, Mantle was assigned as a telegrapher at the Western Union office on the stagecoach line between Corinne, Utah and Helena, Montana. A year later, he purchased the station near Haskell Pass in Flathead County, Montana. Mantle was the Western Union telegraph operator and postmaster, and also purchased an interest in a toll road through Monida Pass.