The Girls Aloud star revealed last year that she had been diagnosed with breast cancer at the age of 38, with former bandmates Cheryl, Nicola Roberts, Nadine Coyle and Kimberley Walsh reaching out to show their support.
She's now shared details of their meet-up at private members club and hotel Soho Farmhouse in 2020, which came around eight years after they'd last been together.
Harding reflects on the reunion in an extract from her upcoming book Hear Me Out published in Glamour.
"We knew it would be low-key and private there, and besides that, it was only two hours from my mum’s house, where I’d been living during my cancer treatment," she writes.
Harding said she had "reservations and fears" about getting together again, in part because of the changes to her appearance due to treatment.
Watch: Cheryl thinks about Sarah Harding's cancer battle 'every waking hour'
"The steroids I’d been taking made me look bloated, and I’d lost my eyelashes due to the chemo," she said. "As the time for the reunion drew closer, I was picturing them all turning up looking fabulous and glamorous, while I looked … well, like I did. Not myself. Not my best."
She explained the fivesome had dinner together and by that point she had "loosened up and started to enjoy [her]self".
"There was much reminiscing. We decided to watch all the episodes of Girls Aloud: Off the Record, which was the documentary-style behind-the-scenes TV show we’d made for E4 in 2006," Harding continued.
A further extract written by Cheryl described it as a "lovely trip" where Harding "was at her best".
It comes as the Celebrity Big Brother winner recently said doctors had told her she won't see another Christmas.
However, Harding said she didn't want to know her exact prognosis, just "comfort" and to be "pain-free".
Watch: Sarah Harding can’t 'look at herself' after mastectomy