In 1875, he joined the New York Stock Exchange and remained a member for forty-four years. He served as president of the Exchange for five years from 1898 through 1902. While he was president of the Exchange, much of his time was devoted to the supervising the construction of the building at 11 Wall Street. When President William McKinley was assassinated in 1901 in Buffalo during the Pan-American Exposition, "it was said that Mr. Keppler's conservative judgment and the trust in which he was held by Wall Street were important factors in preventing a panic and allaying nervous fears." In 1913, he was questioned by Samuel Untermyer during the so-called Pujo Money Trust Investigation.