The 15-year-old dressed for the cooler weather in a navyblazerand trousers and a floral patterned blouse as she walked throughBurghley HouseinStamford.
On the other hand,Sophie, Countess of Wessexwore a red and pink striped skirt and a black T-shirt and could be seen laughing with her daughter from the grandstand at the event.
Later, the 54-year-old was presented with a bouquet of flowers and was seen chatting with sports presenter Clare Balding.
Sophie- who is married toPrince Edward,theEarl of Wessex- and Louise were visitingLincolnshirefor theLand Rover Burghley Horse Trials, an internationally recognisedequestrianand social event.
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Lady Louise is the eldest child of Prince Edward and is Queen Elizabeth II's youngest granddaughter (making her twelfth in line to the throne).
The Wessex family is more private than the other British royal families and they attend very few public engagements compared to Prince William and Kate Middleton.
According to the Daily Mail,this is the third time that the children of Prince Edward and the Countess of Wessex have made public appearances recently.
In July, the Earl and Countess of Wessex were spotted with Louise Mountbatten and her brother James Viscount Severn (the youngest grandchild of Queen Elizabeth II and Prince Philip) at a zoo in Bristol.
In June, the family attended the Trooping the Colour event along with the rest of the royal family for the Queen's birthday.
In a rare public appearance, Lady Louise Windsor was spotted at the Burghley Horse Trials with her mother Sophie Wessex on Sunday.
Sophie and Edward celebrated their 20th anniversary in June.