The victory was confirmed after a protest from the French team - who finished in second place - was dismissed.
Mills carried the British flag at the Tokyo Games opening ceremony with rower Mohamed Sbihi.
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How Hannah Mills and Eilidh McIntyre won sailing gold
Mills and McIntyre have dominated the regatta at Enoshima, winning two races and only twice finishing outside the top four, and they went into the medal race with a 14-point lead.
That meant they only needed to finish in the top seven to clinch gold and they were never in any danger, crossing the line comfortably in fifth.
The victory continued a hugely successful two days for Britain’s sailors, with Mills and McIntyre making it three gold medals after success for Dylan Fletcher and Stuart Bithell in the 49er and Giles Scott in the Finn on Tuesday.
It is the country’s second best Olympic tally after Beijing in 2008, when they won six medals, including four golds.
What medals has Hannah Mills won?
Mills won silver in London 2012 and gold in Rio 2016 with Saskia Clark in the same boat.
She has also won three gold, three silver and two bronze medals in the World Championships.
Who is Hannah Mills’ sailing partner Eilidh McIntyre?
Mills joined forces with 27-year-old McIntyre when Clark retired following the Rio Olympics and the pair marked themselves out as the ones to beat by winning the world championships at Enoshima in 2019.
It is a debut Olympic medal for McIntyre, who follows in the footsteps of her father Mike – the gold medallist in the Star class at the Seoul Games in 1988.
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