She attended the event to support her horse Classicals Euro Star, ridden by equestrian Jason Wood on the day.
The birthday photo shows Charlotte smiling in a Rachel Riley floral print dress.
ZARA TINDALL went to great pains not to steal the limelight from Princess Charlotte
The dress description reads: “A Rachel Riley favourite! Puff-sleeved button-front dress with a vintage floral print in contrast colours on a navy base.
“Trimmed with pink ric-rac trims and buttons.
“Matching dress available for baby girl. Available in a selection of prints.”
Rachel Riley is a British brand, favoured by the Cambridges to dress their children.
Princess Charlotte isn’t wearing any accessories in the photo, and her long hair is worn loose around her shoulders.
Prince William revealed that his daughter had a “lovely birthday”, while touring an engineering firm in the West Midlands.
He said: “She had a lovely day thank you.”
“Last year it was her birthday in lockdown but this year we were able to have one other family over.
“They grow up very fast. It was great fun.”
Source: EXPRESS CO UK