Leo Elwood Allen (October 5, 1898 – January 19, 1973) was an American politician from Illinois. Born in Elizabeth, Illinois, Allen's maternal grandparents were German immigrants and his paternal grandfather was from England. He attended public schools and graduated from the University of Michigan in Ann Arbor in 1923. During the First World War, he served as a sergeant in the 123rd Field Artillery Regiment between 1917 and 1919. He taught school in Galena, Illinois in 1922 and 1923 and was clerk of the circuit court of Jo Daviess County from 1924 to 1932. He studied law and was admitted to the bar in 1930, starting a practice in Galena.