Yet she does not rule out a seventh Games in Rio de Janeiro in 2016 when her own daughter may be a rival for a place in the team.
King is 51, the oldest member of Team GB, and hopes to add gold to the team silver and bronze she won in Athens and Beijing respectively. Nobody should bet against this English housewife, a verger's daughter who returns from a morning schooling her horses on the sands of Exmouth beach in Devon to get the kids off to school and do some baking.
This may read like the stereotypical douce life of a WRVS or National Women's Register stalwart, but don't be fooled. Behind her carefree charm lies one of Britain's most determined and formidably competitive sporting figures. She survived a broken neck to continue her Olympic career on a horse which was once ridden by one of her best friends, the victim of a fatal eventing accident.
And in 1995, when King won European team gold and individual bronze for Britain, she was five-and-a half months pregnant with her daughter, Emily.
No international horsewoman, surely, was more born to ride, so it was no surprise when Emily, at 15, became the youngest member of the British team at last year's European Junior Championships, where she made her debut. "She had a fantastic top-10 result [seventh] in her first-ever team event," King said. "We were the first mother-daughter combination to compete for Britain in our respective teams, which was great."
Perhaps an Olympic double in Rio? "Emily is hoping for a place on the team for the Junior Europeans again this year, so it isn't completely unlikely that we could even be on the same team four years down the line."
It was Emily's young brother who steered mum from thoughts of retirement. "My son, Freddie, is very keen on football," she said, "so when he heard that the 2016 Olympics would be in Rio, he was the first to encourage me not to make 2012 my last Olympics."
Emily already has a special dispensation to contest higher-grade events and, like her mother, she dismisses the threat of death or crippling injury. Two years ago she was airlifted to hospital with multiple injuries after a fall. These included a broken pelvis and a spinal fracture, but she returned to the saddle undeterred, taking her place in the GB eventing squad the following year.
That fall evoked chilling memories for King, who in 2001 suffered an accident akin to that which left the Superman actor Christopher Reeve quadriplegic. "It could have been disastrous," she acknowledged.
King was schooling a young horse at her home near Sidmouth in south Devon when a pheasant shot out of a hedge and spooked the animal, which violently bucked her off. "It could have happened to anybody. It's the hazard of riding horses: you do have falls every now and again. It just depends on the way you fall, whether it ends up bruising you or breaking something.
"This time I landed head-first, on the back of my head. My head went forward sharply and I broke my C5 vertebra and strained ligaments at the back of my neck."
Her mount bolted back to the yard, and King was left in lying the field, alone. "I could feel a tingling sensation in my arms, and I lay there thinking, 'oh dear, that's not meant to be very good'. I quickly moved my legs and thought 'thank goodness'. I had complete body movement, and went to get up. But I couldn't. I couldn't lift my head off the ground. I worked out that if I rolled over on to my side, and got up holding my head, supporting it in my hands when I was standing, that was okay. It felt as though I'd done something between my shoulder blades. That's where the pain was."
King managed the quarter-mile walk back to the yard, where her first thought was for the horse. "I couldn't lift my hands up to take off the bridle. So I phoned a friend and they came and took me to hospital."
They failed to diagnose her broken neck, and she started riding again three days later. When she returned after a week to see the consultant, he looked at the previous week's X-rays. King said: "He told me, 'this is a good broken neck'. He could not believe they hadn't spotted it. He said, 'I hope you haven't been riding'. It could have been absolutely disastrous if I'd fallen off in those few days, but luckily I didn't."
The bone began emerging from the front of her neck, and she required surgery. "I thought that might be the end of my riding career. The surgeon asked if I still wanted to ride. I said I'd love to, and he said, 'well don't ride for eight weeks, and don't fall off for 10'."
King returned to international events in under a year, placing third at Burghley, and the following year won Olympic team silver in Athens. She insists the experience did not affect her nerve at all, and says riders are more vulnerable with young horses. "You see riders galloping round these great big event courses, and think, 'gosh, it's dangerous', but it's less dangerous than producing young horses at home. They're unpredictable and can do naughty things because they haven't been trained and learned what life's all about. But I still find myself thinking, when I'm flying through the air on the occasional fall, 'Oh, my neck! My neck!'"
Yet King made no attempt to influence Emily, who she acknowledges had a head start with a "horsey" mum. On her own childhood King says: "All I wanted to do was ride . . . I was always begging for a pony, as little girls do."
To no avail. She had to make do with riding a pony owned by the local vicar, which she was besotted with, her mother leading her on it along local bridle paths and lanes when she was six. "We didn't have any land. We just lived in a little rented cottage, but friendly farmers let me use their land. The vicar let me use the stable. We lived in a friendly part of the world where everyone was willing to help people."
Leaving school as soon as she was allowed, King got a job with the former European champion Sheila Willcox and learned the horse business. She funded her ambitions working in the yard, then as a chalet girl, and rose to watch leader on the tall ship Sir Winston Churchill. Finally she established her own stables in two cow sheds at a disused farmyard. She looked after other people's animals, giving lessons and trading in horses, but she had to clean houses, cook, garden, and deliver butcher meat to make ends meet.
Her story runs counter to equestrianism's elitist perception. "It's not that way at all," she said. "Some riders may come from wealthy backgrounds, have a kick start, but I assure you, there are many more who are not in that position. Their dream and drive gets them there. Money doesn't win you a medal. You can't just buy a good horse, get on it, and go. It takes years of training, working with that horse."
Zara Phillips would have been an Olympic team-mate much sooner, but for injury to Toytown. As King says, royalty are no different. So few horses reach Olympic standard, a journey which takes years.
Emily had sponsorship at 12, the age at which Mary finally got regular access to a pony. At that age Emily's ambition was to be better than her mother. By 14, the precocious daughter had turned her art and drawing talents to designing a jewellery collection for the owners of one of her horses, Hiho. Yes, it's called Hiho Silver.
But gold is what Mary is fixated on now, and in four years' time, only a brave man might bet against her jumping over Emily to get to Rio.