In 1915 Jennewein became a naturalized U.S. citizen when he was twenty-five years old. Soon afterward he entered the United States Army. In 1916 his tour was cut short when he was awarded an honorable discharge after receiving the Rome Prize, a highly sought-after art award. This allowed him to study at the American Academy in Rome for the next three years; in Rome Jennewein turned his attention to sculpture. By 1928, Jennewein had set up his studio in the Van Nest section of the Bronx where he remained until 1978, the year of his death.