The five-time world champion held off longtime rival Ondrej Synek of the Czech Republic to win in a time of 6 minutes, 57.82 seconds. Synek was a half-length behind in second and Alan Campbell came through late to take the bronze, Britain's sixth medal of the regatta.
Drysdale went into the Beijing Games in 2008 as a huge favourite, but was hit by illness days before the final and struggled to a bronze medal. He had to be helped onto the podium that day after vomiting behind the scenes.
But he made amends four years later, demonstrating his strength to battle a headwind and keep Synek at bay. There was nothing between them at the halfway mark but Drysdale had pushed a length clear by 1,500 metres and held on grimly against the tide, punching his right arm out after crossing the line.
With the men's pair of Hamish Bond and Eric Murray also winning their final on Friday, New Zealand tops the rowing medals table from Britain with three golds with four final races left. Joseph Sullivan and Nathan Cohen won the men's double sculls on Thursday.
Drysdale appeared in danger of missing the London Games when he crashed his bike into a car while out riding before a World Cup regatta in Munich in mid-June. He only sustained minor joint injuries, grazes and bruises.
It was Synek's second straight Olympic silver. The gold-medal winner from Beijing, Olaf Tufte of Norway, failed to reach the final.
Lassi Karonen of Sweden was overhauled in the final 150 metres by Campbell, whose bronze added to the two thirds he has won at the last two world championships.