On Twitter, US swim team assistant coach Ray Looze described the breaststroke final as a battle of good versus evil.
Efimova missed 16 months for doping and tested positive this year for the now-banned substance meldonium. The ugly subplot stirred up images of the old Cold War rivalry between the USA and USSR. Others have likened it to Hollywood's Rocky v Drago.
The final came soon after Chinese swimmer Sun Yang won gold in the men's 200m freestyle final. Yang was branded a "drug cheat" by Aussie Mack Horton, who beat him in the 400m freestyle.
After hitting the wall first, King swam over a lane rope to celebrate with American teammate Catherine Meili, who claimed the bronze.
She did not once acknowledge Efimova in the pool or during their walk from the pool back to the marshalling area ahead of the medal ceremony. King also didn't acknowledge Efimova during a raucous victory celebration but finally, as the swimmers were picking up their Olympic credentials, King gave Efimova a quick pat on the shoulder. Nothing more.
"I basically said what everybody's thinking," King said, adding that other swimmers "were glad I spoke out and had the guts to say that and I appreciate their support."
According to reports, Efimova cried during her post-race interview with a Brazilian television station and complained about how her preparation had been disrupted by her initial suspension from competition.
She was one of seven Russian swimmers initially banned as a result of the World Anti-Doping Agency's investigation into state organised doping in Russia.
Swimming fans showed her little sympathy.
Efimova said she's been treated unfairly, having already served a penalty for a doping violation that occurred while she was training in Los Angeles with one of America's most prominent coaches, Dave Salo.
As for the second positive test, any possible sanctions were put on hold while the World Anti-Doping Agency does more research on meldonium, which was only put on the banned list at the beginning of the year.
"Athletes used to be outside politics," Efimova said. "It's really painful for me that a lot of athletes don't understand that and just watch the TV and accept everything that's said there." She called on them "to swap places with me and understand how I feel".
There was little sympathy from King.
"It just proves you can compete clean and still come out on top with all the hard work you put in behind the scenes, behind the meet, at practice and weight sessions," the 19-year-old King said. "There is a way to become the best and do it the right way."
She told Channel 7 she agreed with Horton's stand against drugs in sport.
"I completely agree with him," she said.
"This is one for the good guys. A 1-3 finish for the USA. We're competing clean and we're going to stay that way. It's still going to bring us success in the pool."
King's victory highlighted another big night for the Americans, who also extended their domination in the men's 100 backstroke with Ryan Murphy's victory and wound up with six medals in all.
Murphy was fourth at the turn, but rallied on the return lap to give the Americans their sixth straight gold medal in the 100 back. Their last loss came at the 1992 Barcelona Games.
For good measure, David Plummer - a 30-year-old Olympic rookie - claimed the bronze.
Hungary's Katinka Hosszu became the first two-time gold medalist at the Olympic Aquatics Stadium, adding the women's 100 backstroke title to her world-record victory in the 400 individual medley.
Hosszu, known as the Iron Lady for her grueling schedule, propped herself on the lane rope and made a heart sign in the direction of her coach and husband, Shane Tusup.
The silver went to American Kathleen Baker.
"I knew that I could win," Hosszu said. "But I was so tired that I told the Hungarians before the race that I could get anything from first place to eighth place."
In another result sure to stir the doping debate, Yang captured gold in the men's 200 free. Two years ago, he served a three-month suspension for taking a banned stimulant.
Yang rallied from his customarily slow start to pass South Africa's Chas le Clos, who went out fast and tried to hang on. It nearly worked.
Yang surged to the front on the final lap, but Le Clos still managed to grab the silver.
Even on a red, white and blue night at the pool, Missy Franklin endured another stunning disappointment. The darling of the London Games failed to qualify for the final of the 200m freestyle, extending a mystifying loss of form since turning pro last year.
Franklin finished last in her semifinal heat with only the 13th-fastest time among 16 swimmers. She actually went slower than she did during the afternoon preliminaries.
As a bubbly, 17-year-old high schooler, Franklin won four golds and a bronze at London, where she competed in seven events. This time, she struggled just to qualify for two individual events and it looks like her only realistic shot at a medal will be on the 4x200 free relay.
"It's so hard," she said, "knowing all the work you put in every day, and then to get here and be so far behind where you feel like you can be."
