Johnson County was formed in 1855 and was named after Vice President Richard M. Johnson. Area Page: Eastern South Nebraska AreaCounty Seat: Tecumseh NebraskaLicense Platte Prefix: 57Census 2010: 5,217Cities Towns in the County: Tecumseh NebraskaCounty Map with Cities: Johnson County MapWebsite - County: Johnson County Nebraska Official WebsiteWebsite - County Fair: Johnson County Nebraska County Fair