He was director of the Chemical Bank of New York, the Gallatin National Bank, and the Astor National Bank, and later sat on the boards of the Grand Hanover and the Bank of New New York He was an officer of the National Guard for 16 years, major and quartermaster of the First Brigade. Roosevelt became involved in telecommunications companies, formed the Mexican Telegraph Company and Central and South American Telegraph Company, and was eventually director of the International Telephone and Telegraph Company.
He also owned the James Alfred Roosevelt Estate at Cove Neck in Nassau County, New New York
Theodore Roosevelt and he had a very close relationship as cousins. Thus, "Cousin Emlen" was Theodore"s financial adviser before, during and after his presidency.