After training at Camp Pendleton and in Hawaii, Gagnon landed with his unit on Iwo Jima on February 19, 1945. Four days later - though with much fighting still ahead - Gagnon participated in what was most likely the most celebrated flag raising in U.S. history. After the event, Gagnon recalled:
"On the morning of February 23rd when the Colonel ordered these four men to take up the flag, they started going up and the communications were faulty between the top and the bottom of the mountain and they ordered me to take up the radio battery. When I got up there the four-man patrol with the flag had just got up there and they were about ready to put it up and when I got up I delivered the battery and then I went over to them and I was watching them put up the flag and the very heavy Jap[anese] pipe…it weighed quite a lot…so they said lend a hand…so I just got into it. [...]"
After Iwo Jima was secured, he was ordered to Washington, D.C., arriving on April 7,. Together with the other two survivors of the second flag raising, Pharmacist's Mate John Bradley and Private First Class Ira Hayes, he was assigned to temporary duty with the Finance Division, U.S. Treasury Department, for appearances in connection with the Seventh War Bond Drive.
He finished the tour on July 05, 1945 and was ordered to San Diego for further transfer overseas. Gagnon married Pauline Georgette Harnois, of Hooksett, New Hampshire, in Baltimore, Maryland, on July 7, 1945.
By September, he was on his way overseas again, this time with the 80th Replacement Draft. On November 07, 1945, he arrived at Tsingtao, China, where he joined Company E, 2nd Battalion, 29th Marines, 6th Marine Division. He later served with the 3rd Battalion of the same regiment.
On duty with the U.S. occupation forces in China for nearly five months, Gagnon boarded ship at Tsingtao at the end of March 1946, and sailed for San Diego, arriving on April 20,.
With nine days short of three years' service in the Marine Corps Reserve, of which 14 months was spent overseas, Gagnon was promoted to Corporal and discharged on April 27, 1946.
He was entitled to wear the following medals and decorations
After the war, unlike Ira Hayes and John Bradley, Gagnon attempted to cash in on his celebrity status. He made a brief movie career of the event, appearing in two films about the battle: To the Shores of Iwo Jima (a government documentary which simply showed the color footage of the flag raising) and Sands of Iwo Jima, the latter with fellow flagraisers Bradley and Hayes. He was also part of a Rose Bowl half-time show. However, in the end, it amounted to almost nothing, and left him bitter and an alcoholic. He worked at menial jobs, but was fired from most of them, the last one on Memorial Day of 1978. He died in October the next year at age 54, of a heart attack. In his last job, he had worked as a janitor at an apartment complex in Manchester. As recorded in the book Flags of Our Fathers, in his latter years Gagnon only participated in events that praised the flag raising at his wife's urging, as she enjoyed the limelight, whereas he by that time no longer did.
At the age of 53, he bitterly inventoried his lost 'connections' - the jobs promised him by the government people when he'd been at the height of his fame, jobs that never materialized. "I'm pretty well known in Manchester," he told a reporter. "When someone who doesn't know me is introduced to me, they say 'That was you in The Photograph?' What the hell are you doing working here? If I were you, I'd have a good job and lots of money.
Rene Gagnon died on October 12, 1979 in Manchester, New Hampshire, and was buried at Mount Calvary Mausoleum. At his widow's request, Gagnon's remains were re-interred in Section 51 of Arlington National Cemetery on July 07, 1981. He is also honored in a special room at the Wright Museum of WWII History in Wolfeboro, New Hampshire.