The pair effectively secured victory as early as Tuesday and only needed to finish the medal race. They duly came in eighth.
A lack of wind meant that the final race had to be frequently postponed, but the duo were finally able to replicate the feat of Giles Scott in the finn, who himself knew he had won a gold the day before the end of his competition due to his insurmountable haul in the competition.
New Zealand's Jo Aleh and Polly Powrie won silver and Camille Lecointre and Helene Defrance of France took bronze.
Rio De Janeiro, Brazil2016 Rio Olympics - Sailing - Preliminary - Women's Two Person Dinghy - 470 - Race 8/9/10 - Marina de Gloria - Rio de Janeiro, Brazil - 16/08/2016. Hannah Mills (GBR) of Britain and Saskia Clark (GBR) of Britain compete
Image credit: Reuters
Mills, 28, and Clark, 36, clinched Britain's 21st gold medal in Rio, coming as it did just an hour after Alistair Brownlee successfully defended his triathlon title ahead of younger brother Jonathan.
The 470, named as such due to the 4.7m length of the dinghy, consists of a 10-race series, with the top 10 boats qualifying for the medal race on a points-per race basis.
The pair's gold medal is Britain's second of the Games in the sailing, following on from Scott's dominant performance in the men's finn class.
Mills and Clark's success takes Britain to the top of the Rio sailing medal list, with Nick Dempsey having also taken silver in the windsurfing.