Genaust was a sergeant in the United States Marine Corps during the Pacific War operating a then-modern and lightweight 16 millimeter motion picture camera which used 50-foot color film cassettes.

He was recommended for the Navy Cross by Norman Hatch, his photo section commander, but the nomination was turned down because Genaust was a cameraman and not an infantryman.

[2] He also shared in the Navy Presidential Unit Citation awarded to the 4th Marine Division for the Battle of Saipan and Tinian (June 15 – August 1, 1944).

There he teamed with Marine still photographer, Private First Class Bob Campbell, and proceeded to take more film action at the base of Suribachi.

The patrol was led by E Company's executive officer, First Lieutenant Harold Schrier, who had volunteered to take over the platoon and raise the battalion's small American flag to signal the volcano was captured.

[2] On the way there, they met Associated Press photographer Joe Rosenthal, who first arrived on Iwo Jima with the 4th Marine Division on February 19 (he went back and forth from a ship each day), but missed the first flag raising on top.

[1] After the three got on top, Genaust with his Bell & Howell Auto Master Filmo 16mm Motion Picture Camera, stood at the left side of Rosenthal, and filmed the second flag being hoisted by six Marines.

[2] Rosenthal became famous for taking the black and white photograph of the second flag raising which appeared in Sunday newspapers on February 25.

On March 3, 1945, Genaust was reported missing in action after he was shot in a large cave at or around Hill 362-A, located on the northern part of Iwo Jima.

He reported that, because of poor lighting conditions on Iwo Jima, Genaust had joined an infantry team clearing caves and tunnels.