They donned their glad rags to celebrate with a 'visit by the Queen' and took selfies with the cardboard cut-out of the royal visitor before enjoying a cream tea and music in the Carriage Circle at the Castle, which was once home to the Bishops of Worcester.
As the Women's Institute is always linked with jam and cakes, local member Louise Thomas baked cupcakes to mark the occasion and members were able to take some home in celebratory boxes.
Marion Inman, the Hartlebury institute's president, said: "It has been a very difficult year in which to celebrate the WI centenary but the tea party held at Hartlebury Castle, by the kind permission of Hartlebury Preservation Trust, really made it a memorable occasion.
"It was an amazing day as the sky was clear blue, the sun shone and the castle gardens looked magnificent.
"It was a great experience and we are very appreciative of all the assistance given to us and allowing the use of the picnic tables and the all important sunshades.
"There was music from through the ages playing in the background which added to the atmosphere of the day.
"Some 28 members attended the celebration tea, and also on display was a tablecloth that had been embroidered with the Hartlebury WI centenary logo and bearing hexagons made by all of the members who had embroidered their names in the centre of each.
"It is hoped that this will be a lasting keepsake of this unusual centenary year.
"Members were also able to view the centenary WI roses, called Inspiration, which were planted in the castle grounds to mark each decade of Hartlebury's existence.
"They were a delight for members to see as they were planted in the autumn in the hope that they would be in full bloom for the garden party.
"This has been a very different year in which to celebrate a centenary but with the help of members and a very active committee it will be a year to remember."