The former TOWIE couple, who split up earlier this year, are filming new scenes for the hit reality TV show together.
Lucy revealed her apprehension about working with him after their rollercoaster relationship ended.
"It's really hard. I was with him for a really long time and of course I still care about him so I'd really like it if we could stay friends."
"I'm a bit worried about having to film scenes with him," she wrote in her new! magazine column. "I'm my own worst nightmare because I wear my heart on my sleeve and I get upset and emotional."
However, a recent photo on Twitter showed that Lucy wasn't letting her and Mario's rocky past relationship get to her while filming in Vegas. The enjoying some girl time with cast members Sam and Billie Faiers and Ferne McCann when posed together at a Las Vegas pool party.
By chance, Mario and Lucy bumped into one another at the American Embassy in London while arranging their visas for the TOWIE trip to Las Vegas.
"I walked in, and to my shock Mario was there, four days before the final of CBB! The show's producers had agreed beforehand to let him out to get his visa."
Lucy told readers that her and Mario were on amicable terms.
"We had a chat for a little while, which was nice," she explained. "I told him he was doing really well in the house and that he should be proud of himself, and he asked if I'd had a nice holiday."
"He said he'd had a lot of time to think about things and me while he'd been in there," she added.