Showalter won U.S. Championship matches against Max Judd (1891/92, +7-4=3), Albert Hodges (1894, +7-6=4), S. Lipschütz (1895, +7-4=3), Emil Kemény (1896, +7-4=4), and John Finan Barry (1896, +7-2=4).
In 1885 he won the championship of the New York Chess Club, and the next year he took part in the international tournament held in London, where he defeated Johannes Zukertort and George Henry Mackenzie.