World War II Leland "Lou" Diamond (May 30, 1890 – September 20, 1951) was a notable member of the United States Marine Corps.

In World War II, Diamond was an expert in mortars and made significant contributions to battles on Guadalcanal, receiving a commendation from General Alexander Vandegrift.

His father, of Jewish ancestry, was the youngest of the famed Diamond Brothers of the North-West Mounted Police (NWMP), who served in the Red Deer District of Alberta in the 1880s.

His ancestors were United Empire Loyalists John Diamond (1759–1845) and Christiana Loyst (1765–1842), from Dutchess County, New York, who fled to Fredericksburgh, Ontario, after the Revolution.

He saw action with the famous 6th Marine Regiment in the battles at Chateau Thierry, Belleau Wood, the Aisne-Marne, St. Mihiel and the Meuse-Argonne.

But the Sino-Japanese conflict, in Diamond's opinion, was "not much of a war," and on June 10, 1933, he returned to the United States, disembarking from the USS Henderson (AP-1) at Mare Island, California.

Two years after his promotion to master gunnery sergeant on July 10, 1939, he was assigned to the Depot of Supplies at Philadelphia to help design a new infantry pack.

Though not a "spit-and-polish" Marine, Diamond proved himself an expert with both 60- and 81-mm mortars, his accurate fire being credited as the turning point of many battles on Guadalcanal.

The Commandant, U.S. Marine Corps has been furnished a copy of this letter, with a request that it be made a part of your official record.After two months on Guadalcanal, physical disabilities dictated his evacuation by air against his wishes.

On July 1, 1943, he disembarked from the USS Hermitage (AP-54) at San Pedro, California, and twelve days later was made an instructor at the MCRD Parris Island, South Carolina.

His death at the Great Lakes, Illinois Naval Training Center Hospital on September 20, 1951, was followed by a funeral with full military honors at Sylvania, Ohio.

Actor Ward Bond portrayed Diamond in an episode of the television series Cavalcade of America entitled "The Marine Who Lived 200 Years."