In April 1945, Laurence was summoned to the secret Los Alamos laboratory in New Mexico by Major General Leslie Groves to serve as the official historian of the Manhattan Project. In this capacity he was also the author of many of the first official press releases about nuclear weapons, including some delivered by the Department of War and President Harry S. Truman. He was the only journalist present at the Trinity test in July 1945, and beforehand prepared statements to be delivered in case the test ended in a disaster which killed those involved. As part of his work related to the Project, he also interviewed the airmen who flew on the mission to drop the atomic bomb on the city of Hiroshima, Japan. Laurence himself flew aboard the B-29 "The Great Artiste", which served as a blast instrumentation aircraft, for the atomic bombing of Nagasaki. He visited the test Able site at Bikini Atoll aboard the press ship "Appalachian", for the bomb test on July 1, 1946.