The "Falling Inn Love" actress, 37, dated the 38-year-old "Masked Singer" host for two years and suspected towards the end of their relationship that he was cheating on her.
Milian told E! News that she confirmed it by hacking into his phone and looking at his messages.
She revealed she guessed his password correctly "on the first try" and gathered her "intel" over the course of several weeks.
"The next thing you know, I was reading. For like a month, I was reading messages," Milian confessed to E! News.
"It was unreal that it was happening. He was living at my house, everything, too. It was just like, 'Wait, under my roof this is happening?' It just kind of blew me away," she added,
Milian explained she was filming a project in Romania, so she had to wait to confront him.
"So I was like, 'I cannot bust him when I am away. I need to bust him in person, so in the meantime, I'm gonna get my intel, make sure that this is for real for real.' And it was for real, so it was enough. I couldn't help it. I was bored, too, so it gave me something to do. It became like entertainment," she explained.
"I wanted to be able to see him in person, go over it — not like it would change anything — but part of me was like, he was my first love," Milian continued.
Cannon was rumored to have been linked to former "America's Next Top Model" winner Eva Marcille during their breakup. He went on to marry Mariah Carey and the two had fraternal twins before splitting in 2016.
He apologized to Milian in a 2012 radio interview. "If I didn't say it before, I do apologize," Cannon said. "I'm sorry for the things that I did. We was young. I was young, I was silly."
Milian had a child with rapper The Dream but their marriage ended in 2011. She's now pregnant with her second child — her first with boyfriend Matt Pokora.