Simon filed divorce papers to dissolve his 11-year marriage to Barney in January 2010 -- less than a month before Barney revealed she was in a new relationship -- and the divorce was finalized in October 2011.
"We started hanging out together more after my separation and one thing led to another. We had been friends for over a year," Barney told RadarOnline at the time.
"We are romantically involved, he is a great guy. It was bound to happen that we both would be dating again after he filed for divorce, it was going to happen sooner or later."
Simon called it "the ultimate act of betrayal" when he learned about the relationship by seeing them together one night in Las Vegas' Hard Rock Hotel while he was under the impression she was back in Los Angeles.