Mosher, South DakotaMosher is an unincorporated community in Mellette County, in the U.S. state of South Dakota.HistoryMosher had its start in 1930 when the railroad was extended to that point. The community has the name of Harmon B. Mosher, a railroad official. A post office was established at Mosher in 1930, and remained in operation until 1974.