McDiarmid was born in July 1869 in Barnesville, Ohio, and spent much of his childhood in Ontario.[2] His family then moved to Cincinnati, where he graduated from Woodward High School, the University of Cincinnati, and the University of Cincinnati College of Law.[2] In addition to practicing law, he played semi-professional baseball as a pitcher.[2]
McDiarmid held front-office positions in Major League Baseball, first as a director of the St. Louis Browns of the American League during 1907–1919,[3] then as secretary of the Cincinnati Reds of the National League.[2] After the 1927 season, McDiarmid succeeded August Herrmann as president of the Reds,[4] presiding over the team during the 1928 and 1929 seasons.[5] During those seasons, the Reds finished in fifth place and seventh place, respectively, in the eight-team National League.[6][7] Late in the 1929 season, Sidney Weil acquired enough shares to become the controlling owner of the Reds.[8]
McDiarmid died in Wyoming, Ohio, in May 1942; he was survived by his wife, three daughters, and two sons.[2] He is buried in Cincinnati's Spring Grove Cemetery.
