Leonard was born on the 25th of February 1947, in Cherry Valley, Illinois, and he grew up hunting, fishing, and trapping along the Kishwaukee River. He was drafted into the US Army just a few days after his 19th birthday in 1966 and he attended basic training at Fort Campbell, Kentucky. Leonard deployed in support of the Vietnam War with Company A, 3rd Battalion, 60th Infantry Regiment, 9th Infantry Division, after completing advanced infantry training at Fort Polk, Louisiana. It was his actions on the 2nd of May 1967, that would later earn him the Medal of Honor. The citation reads:

For conspicuous gallantry and intrepidity in action at the risk of his life above and beyond the call of duty. Sweeping through an area where an enemy ambush had occurred earlier, Sgt. Keller's unit suddenly came under intense automatic-weapons and small-arms fire from a number of enemy bunkers and numerous snipers in nearby trees. Sgt. Keller quickly moved to a position where he could fire at a bunker from which automatic fire was received, killing one Viet Cong who attempted to escape. Leaping to the top of a dike, he and a comrade charged the enemy bunkers, dangerously exposing themselves to the enemy fire. Armed with a light machine gun, Sgt. Keller and his comrade began a systematic assault on the enemy bunkers. While Sgt. Keller neutralized the fire from the first bunker with his machine gun, the other soldier threw in a hand grenade killing its occupant. Then he and the other soldier charged a second bunker, killing its occupant. A third bunker contained a automatic rifleman who had pinned down much of the friendly platoon. Again, with utter disregard for the fire directed to them, the two-men charged, killing the enemy within. Continuing their attack, Sgt. Keller and his comrade assaulted four more bunkers, killing the enemy within. During their furious assault, Sgt. Keller and his comrade had been almost continuously exposed to intense sniper fire as the enemy desperately sought to stop their attack. The ferocity of their assault had carried the soldiers beyond the line of bunkers into the treeline, forcing the snipers to flee. The two men gave immediate chase, driving the enemy away from the friendly unit. When his ammunition was exhausted, Sgt. Keller returned to the platoon to assist in the evacuation of the wounded. The two man assault had driven an enemy platoon from a well-prepared position, accounted for numerous enemy dead, and prevented further friendly casualties. Sgt. Keller's selfless heroism and indomitable fighting spirit saved the lives of many of his comrades and inflicted serious damage on the enemy. His acts were in keeping with the highest traditions of the military service and reflect great credit upon himself and the U.S. Army.

Leonard’s comrade mentioned in the citation was Raymond Wright, who also received the Medal of Honor with Leonard on the 19th of September 1968, from President Johnson. He was discharged from the Army the month before receiving the Medal and he eventually went to work for the supply department at Naval Air Station Whiting Field, Florida. Leonard held a supervisor’s position there for more than twenty years and he was married twice and had three daughters. On the 18th of October 2009, Leonard Bert Keller died at the age of 62 after his motorcycle rolled several times. He had just been honored at the US Navy Fleet Reserve Association in Pensacola, Florida. One month later, he was laid to rest at Arlington National Cemetery: Site 60, Site 9197.