Her success was followed by further Scottish tears of jubilation as an emotional Andy Murray set up a chance to avenge his shattering defeat in the Wimbledon final against Roger Federer by winning a tennis gold medal at the All England Club.
The Dunblane-born star guaranteed Team GB at least a silver medal after beating the Serbian world No 2 Novak Djokovic on Centre Court in straight sets to reach the Olympic men's singles final. He will play the Swiss world No 1 tomorrow.
Earlier, Grainger, a Glasgow-born criminologist, and rowing partner Anna Watkins were pre-race favourites as world champions in the women's double-sculls. The Scot had won silver in the three previous Olympics.
The duo led from the start at Eton Dorney and stormed across the finishing line a length and a half clear ahead of nearest rivals Australia. Grainger's win was the 50th gold medal handed out to a Scot in the history of the Games.
Their victory came on another golden day for Team GB, which took three golds and four bronzes on day seven of London 2012, taking them to third in the medal table behind China and the US.
Victoria Pendleton, the 31-year-old Bedfordshire-born cyclist who retires after London 2012, took Olympic gold in the keirin to add to the sprint gold she won in Beijing in 2008.
Also taking gold in the Olympic Velodrome was the pursuit quartet of Ed Clancy, Geraint Thomas, Steven Burke and Peter Kennaugh who knocked nearly a second off the world record to boot. The Australian team who took silver were nearly three seconds behind.
Prime Minister David Cameron who watched Grainger and Watkins rowing to victory said: "It was emphatic and an absolutely brilliant performance by Katherine and Anna – a privilege to watch."
Commonwealth Games and Sports Minister Shona Robison said: "That was truly outstanding race and Katherine's well-deserved and long-awaited gold is brilliant for her and another boost for Scottish sport."
Grainger's delighted mother, Liz, who was one of the jubilant crowd at Eton Dorney cheering the pair's every stroke indicated her 36-year-old daughter might not be closing her Olympic adventure. "She hasn't ruled out Rio, you know," she said.