Hideshi Niimi was born in 1917 in Paauilo, Hawaii. The family moved back to Hiroshima, Japan. After his parents passed away, he moved back to Hawaii with his brother and sister. He joined the 299th Army National Guard in December 1940 and was an original member of the 100th in F Company. After the bombing of Pearl Harbor, his brother was put in the internment center at Sand Island.
When the war was over, Niimi was a mechanic at the Pearl Harbor Public Works Center. He enjoyed golfing, bowling, and ballroom dancing and became a bonsai instructor at Kaimuki Night School and Moiliili Community Center.