 McDonnell had little interest in parliament, and did not defend the seat in 1929, returning to his business activities.When America entered the Second World War in 1941, McDonnell was appointed Honorary Attaché to Washington where he was able to use his personal and business contacts to assist Lord Halifax, British Ambassador to the United States.He married Ethelwyn Sylvia Arthur Jones, the daughter of Henry Arthur Jones, on 13 December 1913.