Stephen was born on the 5th of March, 1944, in Bethpage, New York, and while not much information is known about his early life, it is known that he graduated from Chaminade High School in 1961 and then from Spring Hill College four years later. He was a First Lieutenant with the 2nd Battalion, 35th Infantry Regiment, 25th Infantry Division, when he was deployed to Vietnam and it was his actions on the 12th of March, 1967 that would 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. While leading the 3d Platoon, Company A, on a flanking maneuver against a superior enemy force, 1st Lt. Karopczyc observed that his lead element was engaged with a small enemy unit along his route. Aware of the importance of quickly pushing through to the main enemy force in order to provide relief for a hard-pressed friendly platoon, he dashed through the intense enemy fire into the open and hurled colored smoke grenades to designate the foe for attack by helicopter gunships. He moved among his men to embolden their advance, and he guided their attack by marking enemy locations with bursts of fire from his own weapon. His forceful leadership quickened the advance, forced the enemy to retreat, and allowed his unit to close with the main hostile force. Continuing the deployment of his platoon, he constantly exposed himself as he ran from man to man to give encouragement and to direct their efforts. A shot from an enemy sniper struck him above the heart but he refused aid for this serious injury, plugging the bleeding wound with his finger until it could be properly dressed. As the enemy strength mounted, he ordered his men to organize a defensive position in and around some abandoned bunkers where he conducted a defense against the increasingly strong enemy attacks. After several hours, a North Vietnamese soldier hurled a hand grenade to within a few feet of 1st Lt. Karopczyc and two other wounded men. Although his position protected him, he leaped up to cover the deadly grenade with a steel helmet. It exploded to drive fragments into 1st Lt. Karopczyc's legs, but his action prevented further injury to the two wounded men. Severely weakened by his multiple wounds, he continued to direct the actions of his men until he succumbed two hours later. 1st Lt. Karopczyc's heroic leadership, unyielding perseverance, and selfless devotion to his men were directly responsible for the successful and spirited action of his platoon throughout the battle and are in keeping with the highest traditions of the U.S. Army.

Stephen Edward Karopczyc was 23 years old when he died and his family received his Medal of Honor on the 9th of January, 1969 from the Secretary of the Army. He is buried in the Long Island National Cemetery in East Farmingdale, New York: Plot Distinguished Service Section, Site 5-A, and his name appears on the Vietnam Veterans Memorial Wall: Panel 16E, Line 69.