The royal, who turns 31 later this month, shared a new photograph of August on Instagram to mark the occasion.
The adorable image showed baby August lying on his back on a striped blanket in the garden, surrounded by daffodils.
He was wearing whale-print leggings from Kent-based business The English Beagle, a white knitted jumper and a matching hat, with adorable white fluffy bunny shoes from Doudou et Compagnie finishing off his spring outfit.
WATCH: Princess Eugenie reveals sweet meaning behind baby August's name
Eugenie captioned the post: "I'm so excited to be August's mum and as you can see I'm enjoying my first Mother’s Day.
"I'm also celebrating my beautiful Mumma with this picture of us from March 1990. You’ve taught me so much. Happy Mother’s Day to all."
The second photo showed Sarah Ferguson holding baby Eugenie, who was wrapped in a white blanket as she slept soundly.
Princess Eugenie shared this adorable new photo of her son August on Mother's Day
August Philip Hawke Brooksbank was born on 9 February at the Portland Hospital in London, but his name wasn't confirmed by his parents until 11 days after his birth.
Eugenie and husband Jack Brooksbank shared some gorgeous images with their newborn on social media on the day, taken by their "wonderful" midwife.
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His name pays homage to Queen Victoria's husband Prince Albert, whose full name was Francis Albert Augustus Charles Emmanuel.
Meanwhile, August's middle names, Philip and Hawke, are tributes to his great-grandfather, the Duke of Edinburgh, and Jack's great-great-great-great-great-grandfather, Rev. Edward Hawke Brooksbank.
Eugenie and Jack welcomed their first child in February
Eugenie and Jack, who married in October 2018, are currently living at Prince Harry and Meghan's Windsor home, Frogmore Cottage.
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