Robert Modrzejewski was born on July 3, 1934, in Milwaukee, Wisconsin. He enlisted in the U.S. Marine Corps Reserve on May 2, 1955, and was commissioned a 2d Lt in the U.S. Marine Corps through the Platoon Leaders Class upon graduation from the University of Wisconsin on June 18, 1957. After completing The Basic School at MCB Quantico, Virginia, Lt Modrzejewski remained at the school as an instructor until May 1958. His next assignment was as a Platoon Leader and S-2 Officer with 3rd Battalion, 3rd Marine Regiment of the 3rd Marine Division at Camp Pendleton, California, from June 1958 to September 1959, followed by service as Equipment Officer with the Landing Support Company, 2nd Service Battalion of the 2nd Marine Division at Camp Lejeune, North Carolina, from September 1959 to May 1960. Lt Modrzejewski served as Pathfinder Team Leader of the 2nd Force Reconnaissance Company at Camp Lejeune from May to December 1960, and then as Pathfinder Team Leader with Sub Unit One, HMR(L)-262 with Marine Air Group 26 at MCAS New River, North Carolina, from December 1960 to May 1961. He then returned to the 2nd Force Reconnaissance Company as Pathfinder Team Leader and Parachute Pathfinder Team Leader from May 1961 to May 1962, followed by service as Assistant Officer in Charge of Marine Corps Recruiting Station Cincinnati, Ohio, from May 1962 to May 1965. His next assignment was as Executive Officer of Company E with the Officer Candidate School at MCB Quantico from May to August 1965, and he then attended the Amphibious Warfare School from August 1965 to February 1966. Capt Modrzejewski deployed to South Vietnam as Commanding Officer of Company K, 3rd Battalion, 4th Marine Regiment of the 3rd Marine Division in June 1966, and he later served as Battalion S-3, as a Company Commander, and as the Command Operations Center Watch Officer with Headquarters Company, 3rd Marine Division, before returning to the United States in June 1967. His next assignment was as Commanding Officer of the Marine Barracks at the U.S. Naval Academy from June 1967 to January 1970, and he then attended the Armed Forces Staff College at Norfolk, Virginia, from January 1970 to June 1971. Maj Modrzejewski served with Headquarters and Service Company of the 1st Marine Brigade at MCAS Kaneohe, Hawaii, from 1971 to 1974, and then completed his Master's degree in Education at Pepperdine University in Los Angeles, California, in 1976. Col Modrzejewski served as Commanding Officer of the Infantry Training School at Camp Pendleton from May 1981 to July 1983, and then as the Assistant Chief of Staff, G-2/3, at MCRD San Diego, California, from August 1983 until his retirement from the Marine Corps on August 29, 1986.

His Medal of Honor Citation reads:

For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty. On 15 July, during Operation HASTINGS, Company K was landed in an enemy-infested jungle area to establish a blocking position at a major enemy trail network. Shortly after landing, the company encountered a reinforced enemy platoon in a well-organized, defensive position. Maj. Modrzejewski led his men in the successful seizure of the enemy redoubt, which contained large quantities of ammunition and supplies. That evening, a numerically superior enemy force counterattacked in an effort to retake the vital supply area, thus setting the pattern of activity for the next 2 1/2 days. In the first series of attacks, the enemy assaulted repeatedly in overwhelming numbers but each time was repulsed by the gallant marines. The second night, the enemy struck in battalion strength, and Maj. Modrzejewski was wounded in this intensive action which was fought at close quarters. Although exposed to enemy fire, and despite his painful wounds, he crawled 200 meters to provide critically needed ammunition to an exposed element of his command and was constantly present wherever the fighting was heaviest, despite numerous casualties, a dwindling supply of ammunition and the knowledge that they were surrounded, he skillfully directed artillery fire to within a few meters of his position and courageously inspired the efforts of his company in repelling the aggressive enemy attack. On 18 July, Company K was attacked by a regimental-size enemy force. Although his unit was vastly outnumbered and weakened by the previous fighting, Maj. Modrzejewski reorganized his men and calmly moved among them to encourage and direct their efforts to heroic limits as they fought to overcome the vicious enemy onslaught. Again he called in air and artillery strikes at close range with devastating effect on the enemy, which together with the bold and determined fighting of the men of Company K, repulsed the fanatical attack of the larger North Vietnamese force. His unparalleled personal heroism and indomitable leadership inspired his men to a significant victory over the enemy force and reflected great credit upon himself, the Marine Corps, and the U.S. Naval Service.