The 34-year-old Dubliner won gold at 60kg at the pandemic-delayed Tokyo Olympics three years ago and defeated China's Yang Wenlu by split decision to make it two golds in a row.
It was Ireland's fourth gold in all sports in Paris.
Harrington said she wanted to go out on a high after making a piece of history as the only Irish boxer ever to retain an Olympic title.
"I'm done and dusted. I'm going to go out a champion, that's the way I want to go out. I don't want to go out and lose a fight and then retire. I'll retire happy," she said.
Harrington was among the favourites from the very start of the boxing in the French capital but Yang was the top seed, with the Irish boxer seeded third.
Harrington had strong backing throughout from the vocal Irish fans who outnumbered the Chinese spectators.
Having been the more aggressive of the two fighters, she leapt into the arms of her corner and danced around the ring after the judges' decision was delivered.
She then knelt down and banged the canvas, before leading the Irish celebrations from the middle of the ring.
A former world and European champion, Harrington's triumph in Tokyo propelled her to prominence back home.
She suffered her first defeat in three years in April, only to steamroller her way into the Paris final, and ultimately to a deserved gold.
"This is the stuff of dreams. Very few people in the world get to experience this," said Harrington.
She added: "To have the support of the nation. It is just amazing. The people here today lifted me. They made me feel like I wasn't tired, and I was exhausted."
The Paris boxing was held initially at the North Paris Arena but has moved to Roland Garros, better known for hosting Grand Slam tennis, for the remaining days of the event.
In other bouts at the 15,000-seat Court Philippe-Chatrier, the main arena at Roland Garros, the United States' wait for a first men's gold since Andre Ward in 2004 goes on after a narrow defeat for Omari Jones in the 71kg division.
Uzbekistan's Asadkhuja Muydinkhujaev prevailed in a split decision over Jones. He will now face off for the gold with Mexico's Marco Verde, who squeezed past Britain's Lewis Richardson.
Losing semi-finalists take home bronze.
