The former Spice Girls singer dated Williams, who was then a member of Take That, for a brief period in 1997.
She told Woman's Own magazine — reported via Metro — that she has spoken about the relationship in her latest memoir Who I Am.
When asked whether she contacted Williams to let him know ahead of time about the details set to appear in her book, Mel replied "nah, sod him".
Watch: Mel C says there was very little mental health support in Spice Girls era
The 48-year-old added: "I’m not warning him about anything the way he’s behaved over the years. Like all of us, relationships come and go.
"It really hurts at the time, but once you’re over it, you’re over it and you move on. Still, it was nice to put the truth out there."
Mel has had numerous relationships throughout the years, including with Red Hot Chilli Peppers frontman Anthony Kiedis, and recently split with music executive Joe Marshall — her partner of seven years.
In 2009, she gave birth to her first daughter Scarlet with her ex, property developer Thomas Starr.
Earlier this month, Mel revealed she was sexually assaulted during a massage at a hotel in Turkey the night before the first Spice Girls concert in 1997.
She told the How to Fail podcast: "I felt violated. I felt very vulnerable. I felt embarrassed. And then I felt unsure — have I got this right? I was in an environment where you take your clothes off with this professional person."
Mel said she "buried" the incident for years and years before deciding to open up about it in the book.
She added: "Of course, I had to think 'well, do I want to reveal this?'. And I just thought, actually, I think it's really important for me to say it, and to finally deal with it and process it."
Watch: Mel C reveals she was sexually assaulted before Spice Girls concert