The imaginative four-year-old was seen mucking about with a horse crop and charging across the field with it as she appeared to pretend it was a javelin.
She was also seen bringing a smile to the face of her minder as she stood at the side of the field with a towel over her head.
In another sweet snap, the adorable child was seen lounging in the sun chewing on her sunglasses as she waited for her mother to wind things up at the Whately Manor International Horse Trials at Gatcombe Park.
With Mia was her new little sister, Lena, who Zara gave birth to just three months ago.
The tot remained wrapped up tight as the typically scorching summer weather gave way to a characteristically chillier September morning as autumn began to creep in.
For the trials, equestrian Zara, 37, wore figure-hugging white trousers and a blue backlit and gloves as she showed off her skills on a horse.
Though her 39-year-old husband Mike, a former rugby star, was not present, her mother and daughter of the Queen, Princess Anne, was.
Princess Anne, 68, kept herself warm in a patterned jumper, green body warmer and dark trousers as she took her granddaughter Mia out with her dog, a pit bull terrier.
Mia, who no-doubt gets her confidence from her mother, was seen taking command of the pooch as she and her grandmother walked the animal.
They were pictured getting into a car together, while the curious dog stuck its head out of the window.
Today’s day out was the first time baby Lena was seen in public following her birth on June 18.
Zara has recently opened up about the pain of suffering two miscarriages.
Despite saying the pain made her marriage stronger, the granddaughter to the Queen gave an honest account of how it affected her.
She told the BBC: “I think that’s the hardest thing in our situation, is that everyone knew.
“And very much when things like that happen, normally it’s just your family and friends, but unfortunately everyone knew about it.”
Lena came into the world just two months after cousin Prince William and wife Kate announced the birth of their newest child, Prince Louis. Prince Louis was born on April 23.