Lucas, 40, revealed few details about her (like, uh, her name), but he did tell Leno about the special Big Sur retreat that made him realize his girlfriend was the one.
"I hear two big breaths, and six feet above me, I see the very slight glint of the eyes of a huge [mountain lion] and I literally proceed to do what I had just read in the instructions," he explained on Friday's show. "And I started screaming and yelling and literally being as crazy as I could be, it padded off and just slowly went away and I worked my way into the trailer and I got in and I decided at that moment that this woman was the woman for me."
Lucas, best known for roles in Sweet Home Alabama and A Beautiful Mind, was previously linked to Rachel McAdams and Salma Hayek. He next stars in the TV adaptation of The Firm, which premieres Sunday at 9/8c on NBC.
Watch Lucas talk about his new fiancée: