The countess was also greeted more traditionally by the Lord Lieutenant of Berkshire Mary Bayliss and the mayor of the Royal Borough of Windsor and Maidenhead Cllr Asghar Majeed.
It was Norden Farm's first royal visit and operations manager Rachel Entwistle said: "It was really exciting and an honour to have her here."
A full house of theatre-goers in black tie attended the event which was also part of the Queen's Diamond Jubilee celebrations.
A set of replica crown jewels was especially made for the show which follows the life of the first Queen Elizabeth and runs until Saturday.
Opera member Bernard Hawkes said: "It went wonderfully well.
"First nights can always be a bit twitchy but it went off like a dream and Her Royal Highness made mention that she thoroughly enjoyed it."
See photos from the event below.
Top Ten Articles
The Met Office has issued a yellow alert for Windsor and Maidenhead, Slough and Wokingham, warning that rain moving eastwards across the south of the country may turn to snow in some places.