The 87-year-old also named The Common Years as her favourite piece of work she has written, after being made a dame for her services to literature and charity on Tuesday.
She is chiefly known for her Rutshire Chronicles, which focus on scandal and adultery in upper-class society and an aristocratic character called Rupert Campbell-Black.
The series has seen a string of bestsellers with titles such as Riders, Rivals, Polo, Mount! and The Man Who Made Husbands Jealous along with her most recent work, 2023’s Tackle!.
The investiture ceremony at Windsor Castle was the first carried out by Charles since the King was diagnosed with cancer.
Asked how it felt to receive the honour, Dame Jilly told the PA news agency: “Orgasmic! Just terribly exciting and nice.
“I never dreamed in a million years it would happen and it just has, so it’s lovely.”
Dame Jilly said it was “lovely” to see the King again after the pair previously met at His Majesty’s wedding.
Tackle! was nominated in the best fiction category at this year’s British Book Awards in March, almost 50 years since Dame Jilly first began publishing fiction work in 1975.
Dame Jilly joked the secret to her longevity and her success was “luck” and said she always wanted to write “happy books”.
She said: “The only thing I wanted to do in life was to cheer people up and people can get quite depressed, so I do like to tell lots of jokes.”
A Disney+ series based on her 1988 novel Rivals, featuring British actor Alex Hassell as Campbell-Black, is currently in production.
The eight-part adaptation will also star David Tennant, Aidan Turner, Emily Atack and Danny Dyer, with first-look pictures released last week.
Dame Jilly said the series was “absolutely sensational” and that the cast were “all fantastic”.
It was also announced on Thursday that her fellow novelist, Dame Shirley Conran, had died aged 91 shortly after receiving her honour while in hospital.