After three successive silvers, Grainger claimed the top prize in her home Games on the waters of Eton Dorney.
The Scot and double scull partner Anna Watkins led from the start and were always in control of the race.
For Grainger, it caps a remarkable Olympic journey, and she couldn’t hide her relief.
“I’m very proud of the three silvers that have gone before, and every Olympics I have gone a little closer to the gold medal and never quite got it right,” said the 36-year-old.
“It has made the journey by far deeper emotionally and it has made this matter so much more.
“Having come close so many times it has made the last 12 years incredibly special and to finally get it right feels like job well done.”
Watkins, 29, and Grainger have never been beaten since teaming up in 2012, and were widely tipped for gold.
Watkins said: “The Australians put in a better start than I have done so far this year so we knew we had a race on our hands.
“But we started moving out and there was absolutely no way we were going to let it go. I knew we would do it.”
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BIOGRAPHY: Lucas Torreira