The Court Philippe-Chatrier, the main arena at Roland Garros, had already reached fever-pitch following Imane Khleif’s semi-final win over Thailand’s Janjaem Suwannapheng.
Harrington entered the stadium to a raucous reception by the Irish fans, who dominated the home of French tennis in their numbers with flags and colour.
It was a controlled start by Harrington, who let Yang come on to the front foot but the Irish fighter was measured in her approach.
Harrington landed a superb blow in the final action of the round, which briefly caused the Chinese fighter to stumble, and she was awarded the round by all but one judge.
The second round followed a similar vein, but Harrington stumbled onto the canvas midway through before unleashing a barrage of punches in an immediate response.
The scores were split in the second round, but it was Harrington who held advantage heading into the decider with three judges in her favour, while two more scored the fight level.
Yang had to come forward in the final round, but again the composure and control by her opponent shone through, as the Irish crowd rose to their feet in the final 10 seconds of the bout.
After the bell both fighters shared an embrace, before Harrington walked around the ring with her hands in the air, before dancing with the joyous Irish support who had travelled to the north west of Paris in their thousands.
When Harrington won Olympic gold in Tokyo she was awarded the freedom of Dublin.
Now, she has enhanced her reputation as one Ireland’s sport greats.
Along with rowers Paul O’Donovan and Fintan McCarthy, who defended their double lightweight sculls title, and hammer thrower Pat O’Callaghan, who won gold in 1928 and 1932, Harrington is the only other Irish athlete to defend their Olympic title.
Harrington joins swimmer Daniel Wiffen, gymnast Rhys McClenaghan and rowers O’Donovan and McCarthy as Ireland’s gold medallists at a historic Games in Paris.