The Irish model - who is currently married Made In Chelsea star Spencer Matthews - was married to the 40-year-old former Westlife singer for three years before they split in 2012.
Williams, 34, said of her first marriage on new podcast Spencer and Vogue: “It makes me a little bit sick in my mouth.”
She was responding to 32-year-old Matthews’ rant that being reminded she was married to someone else before him made him feel “c**p”.
He confessed: “Well you were married... Every time I go into the shop by the way I have to see your marriage certificates because like we needed them for a mortgage and I just have to look at them. Like what the f**k is this c**p. And it is unearthed everyday. I look in the safe, which is most days.”
Williams added: “I’m so glad I don’t have to look at the safe. Sometimes I forget it ever happened. Let’s bin that one."
Williams began dating McFadden in 2011. They married in September 2012 and announced their split in July 2015.
She became engaged to Matthews in 2018 and they married in June that year.
They are parents to 23-month-old son Theodore and daughter Gigi, born in July this year.
Matthews, who previously revealed he has become sober since becoming a father wrote on social media: “"Wow, we officially have CHILDREN!! So much has happened in the last three years. Real positive change, a son and now a daughter.
"I couldn’t be more thankful to my incredible wife @voguewilliams for all that you do for our family, in particular the laborious pregnancies which us men will never fully understand.
"I have always dreamt of being a great father and because of you, that is now achievable. Feel lucky ever day to be married to you [sic]."
Last December, McFadden announced he would be marrying his PE teacher girlfriend Danielle Parkinson.
The couple, both 39, have been having IVF treatment for over a year and recently revealed their heartbreak after losing a baby through miscarriage.