The eventer said she is “shocked and devastated” by the news.
Mary received the yellow card at Bramham (6-8 June) for dangerous riding on MHS King Joules.
Last October she was given a card for the same offence, at Le Lion d’Angers, where she was riding Cavalier Venture.
Anyone receiving 2 yellow cards for the same offence in a 12-month period is given an automatic 2-month ban by the FEI.
Mary wrote in her blog that she had some “embarrassing news”.
“The horse I rode at Bramham was disappointingly strong and unruly across country,” she wrote.
“I pulled him up half way around the cross-country course and retired after we had jumped the first part of a rail, ditch, rail combination.
“He dropped his back legs on the first rail and the frangible pin broke.
“It felt dangerous and neither of us were enjoying ourselves so I thought it best to stop before we had an accident.
“The ground jury have interviewed me and said I should have pulled up at the fence before. They said because I am a high profile rider/role model for younger riders I should have been more sensible, and I received a yellow card for dangerous riding. I was completely shocked and devastated.”
She added that a couple of her younger horses will be ridden in June and July by her daughter Emily.
“I love my horses and love the sport, I am very sorry this has happened.”
Her ban runs until 9 August.