Onnie Lattu entered military service in the U.S. Navy in 1930 after graduating from the University of California at Berkeley. In August 1940, he was sent to the American Embassy, Berlin, Germany, as Assistant Naval Attach, also being attached to American Legations in Madrid, Spain; Rome, Italy; Stockholm, Sweden; and Vichy, France. After the United States entered World War II in December 1941, he was interned with other military attaches in Germany until the diplomatic exchange of internees was arranged in May 1942. He retired as a U.S. Navy Rear Admiral.