I looked in the Marriage Records of Knox Co., Ohio 1808-1875 and there is a marriage for John M. Cook and Sarah Johnston 6 Aug 1934. James Scott was the minister.
There were so many Johnston/Johnson families in Washington Co., PA in the early 1800s that it is next to impossible to try and sort them out. There was also a clan of Johnston/Johnsons that settled in Knox Co., Ohio in the early 1800s that came from New York which makes things even more confusing. Good luck.