Mercer Johnston worked at a law clerk in San Antonio, Texas, from 1889 to 1991, before studying law at the University of Virginia, in Charlottesville, Virginia. He then entered the University of the South in Sewanee, Tennessee, and was ordained an Episcopal priest, serving at Grace Church in New York City from 1998 to 1900. From 1900 to 1903 he was Chairman of the faculty, chaplain, and teacher at West Texas Military Academy, San Antonio, Texas, and also served rector of Saint Paul’s Church and minister in charge of Saint John’s Church there. From 1903 to 1908 he was rector of Cathedral Parish of Saint Mary and Saint John, in the Philippine Islands. He returned to the United States and served as business manager of West Texas Military Academy, in San Antonio, Texas, and served Rector of Trinity Lutheran Church in Newark, New Jersey. In 1917 he went to France as a member of the Y.M.C.A., serving with the French Army in France; as a chaplain with the American Expeditionary Forces; and then as a member of the National Committee for Relief of Political Prisoners until 1924. After multiple other assignment, he retired from active ministry in June 1944.