After clinching victory, Harrington said she was hanging up her gloves.
"The last hurrah, I'm done. I've always said I want to retire a champion, that's it," Harrington told Irish national broadcaster RTE.
"It just gives hope to all these young kids, all these teenagers (pointing at the crowd). It gives hope to the people of Ireland but this one was for me."
Brazil's Beatriz Ferreira and Taiwan's Wu Shih-Yi claimed the two bronze medals on offer after losing their semi-final bouts against Harrington and Yang respectively.
Ex-world champion Harrington, who impressed in a convincing win over Tokyo silver medallist Beatriz Ferreira in the semi-finals, faced Yang in the light welterweight final at the 2016 World Championships in Kazakhstan.
On that occasion, the Chinese boxer won, but eight years later and fighting at a different weight, Harrington was a much tougher proposition.
She won the first two rounds with relative ease, gliding around the ring with her silky footwork and peppering her opponent with shots from range.
Perhaps hoping to sign off her amateur career with a bang, Harrington was less careful in the final round and adopted a more aggressive approach, which resulted in her losing the round, though the damage was already done for Yang.
A delighted Harrington, roared on by a sea of Irish fans at Court Philippe-Chatrier, embraced her coaches in her corner before falling to her knees in the ring and thumping the mat.
