The royal shared a series of sweet photographs of her mother to mark the occasion, including several taken in the garden at the family's Windsor home, Royal Lodge.
Eugenie also included a recent mother-daughter selfie, as well as an adorable photo of Sarah with both daughters, Beatrice and Eugenie, dressed in matching floral dresses, taken in 1992.
WATCH: Sarah Ferguson thanks fans for birthday wishes
Eugenie captioned the carousel of snaps: "Happy Birthday to my dear mumsy… You are one of a kind. There were too many pictures to choose from that show how completely unique you are."
The Duchess also gave royal fans a glimpse of her beautiful dressing table as she thanked fans for their birthday wishes in an Instagram video. Wearing a green dress, Sarah waved at the camera and said: "Hi everybody, I just wanted to say a very quick thank you so much for all of my wonderful birthday messages - 61, still red-headed and not out!"
Sarah is set to become a grandmother for the first time when Eugenie gives birth to her first child in early 2021. In the past two years, she's seen both her daughters walk down the aisle, with Beatrice marrying Edoardo Mapelli Mozzi in a private Windsor ceremony in July.
Eugenie's sweet Instagram post
The Duchess, who has remained good friends with her former husband Prince Andrew since their divorce in 1996, has been keeping young children entertained with her YouTube channel Storytime with Fergie and Friends, which she launched at the start of lockdown.
Eugenie also celebrated her second wedding anniversary with Jack Brooksbank on Monday 12 October.
The Princess shared a video montage of unseen wedding photos and holiday snaps of herself and her husband on Instagram to mark the occasion, writing: "Been the best 2 years married to you and 10 years with you in total... happy memories always my dear Jack".
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