Rosenthal was rejected by the U.S. Army as a photographer because of poor eyesight. In 1941, he attended the University of San Francisco and joined the staff of the Associated Press (AP). In 1943, he joined the United States Maritime Service as a photographer and served as a warrant officer documenting life aboard ship in the British Isles and North Africa. In 1944, he rejoined the Associated Press and followed the United States Army and U.S. Marine Corps in the Pacific Theater of Operations as a war correspondent at Hollandia, New Guinea, Guam, Peleliu, Angaur, and Iwo Jima.