The star marked the day (17 November) by sharing a montage of video clips of Harding, who died in September at the age of 39 after battling cancer.
The video included clips of the group when they were trying out for Girls Aloud on TV show Pop Stars: The Rivals and others of them performing later in their career.
“Memories may be all we’re left with, but they’re enough to last a lifetime,” wrote Cheryl on Instagram, adding the hashtag “#Sarahs40th”.
Band member Nicola Roberts shared a string of photos of her and Harding together.
“I know you’re somewhere making trouble live one,” she wrote on Instagram.
“Happy Earth birthday Sarah. Love, miss and thinking about you all the time.
“I feel your energy like you’re still here.
“Wish we could have ran riot around town like we used to for your big 4.0 today! Cause a supernova up there electric girl.”
Sharing a picture of Harding on a night out, Kimberley Walsh said it felt “surreal”.
“Today is a very strange day,” she wrote.
“We have celebrated so many birthdays together over the years so this feels very surreal. I hoped with all my heart that we would be celebrating our 40th’s together too as we planned.
"You ALWAYS brought the party and I have spent today reflecting and reliving all the amazing memories we made.”
Harding revealed last August that she had been diagnosed with breast cancer and that it had spread.
Read more: Sarah Harding's 12 greatest moments
Her mother Marie announced on 5 September that she had passed away, saying her “beautiful” daughter was “a bright shining star”.
Watch: Sarah Harding 'signed up for Girls Aloud tour' before untimely passing