Francis was born in Whitehall, New York, on the 22nd of April 1912, and he grew up in nearby Salem, New York. After graduating Granville High School, he worked for a furniture factory in addition to farm work before enlisting in the US Army in March of 1942. He deployed to Europe during World War 2 with Company K, 109th Infantry Regiment, 28th Infantry Division, and it was his actions on two separate dates and locations that would later earn him the Medal of Honor. The citation reads:

He fought gallantly in Luxembourg and Germany. On 12 September 1944, Company K began fording the Our River near Kalborn, Luxembourg, to take high ground on the opposite bank. Covered by early morning fog, the 3d Platoon, in which TSgt. Clark was squad leader, successfully negotiated the crossing; but when the 2d Platoon reached the shore, withering automatic and small-arms fire ripped into it, eliminating the platoon leader and platoon sergeant and pinning down the troops in the open. From his comparatively safe position, TSgt. Clark crawled alone across a field through a hail of bullets to the stricken troops. He led the platoon to safety and then unhesitatingly returned to the fire-swept area to rescue a wounded soldier, carrying him to the American line while hostile gunners tried to cut him down. Later, he led his squad and men of the 2d Platoon in dangerous sorties against strong enemy positions to weaken them by lightning-like jabs. He assaulted an enemy machine gun with hand grenades, killing two Germans. He roamed the front and flanks, dashing toward hostile weapons, killing and wounding an undetermined number of the enemy, scattering German patrols and, eventually, forcing the withdrawal of a full company of Germans heavily armed with automatic weapons. On 17 September, near Sevenig, Germany, he advanced alone against an enemy machine gun, killed the gunner and forced the assistant to flee. The Germans counterattacked, and heavy casualties were suffered by Company K. Seeing that two platoons lacked leadership, TSgt. Clark took over their command and moved among the men to give encouragement. Although wounded on the morning of 18 September, he refused to be evacuated and took up a position in a pillbox when night came. Emerging at daybreak, he killed a German soldier setting up a machine gun not more than five yards away. When he located another enemy gun, he moved up unobserved and killed two Germans with rifle fire. Later that day he voluntarily braved small-arms fire to take food and water to members of an isolated platoon. TSgt. Clark's actions in assuming command when leadership was desperately needed, in launching attacks and beating off counterattacks, in aiding his stranded comrades, and in fearlessly facing powerful enemy fire, were strikingly heroic examples and put fighting heart into the hard-pressed men of Company K.

Almost a year later, Francis received the Medal of Honor from President Truman in a ceremony at the White House. He left the Army around the same time and returned home to Salem and continued farming and working at the furniture factory. He went on to become a town supervisor, chairman of the Washington County Board of Supervisors, and mayor of Salem. Francis J Clark died on the 20th of October 1981, at the age of 69 and he is buried with his wife in the Evergreen Cemetery in Salem, New York: Section J, Lot 14.