Brighton resident Shelia Coldicott, who has volunteered for Samaritans for 32 years, was one of 39 guests invited to the event, which celebrates the charities and organisations that Prince Charles supports.
The volunteers are attending the prestigious party after being nominated for their commitment and dedication by colleagues.
Shelia said: "It’s such an honour to have been chosen. I’ve heard about garden parties and seen them on the TV and suddenly I’m going to be part of one, alongside other Samaritans volunteers.”
Although Prince Charles' actual birthday falls in November, Buckingham Palace holds the event earlier in the year to take advantage of the better weather.
The volunteers will be joined at the Palace by Samaritans CEO Ruth Sutherland, and its chair of trustees, Jenni McCartney.
The pair said: “Thanks so much to Prince Charles for everything he has done to support Samaritans. This promises to be a wonderful event and a great birthday celebration. It’s also an important way for us to pay tribute to just some of wonderful and dedicated volunteers.”
Samaritans volunteers will be presenting Prince Charles with a personalised photo gift, which aims to represent the number of calls for help Samaritans has answered since he became Patron. Over the 19 years as Patron, Samaritans has responded to approxaimaltely 96.8 million calls for help.