The 50-year-old actor surprised Mann during Wednesday's episode of Busy Tonight as the 47-year-old actress recalled her past crush on him.
Mann told host Busy Philipps she thought she "would marry Brendan" while filming the 1997 movie. Mann played George's (Fraser) love interest, Ursula Stanhope, in the film.
"He was just delightful and funny. In the movie I'm supposed to fall in love with him, and so I just fell in love with him in real life."
Mann told then-boyfriend Judd Apatow, whom she married in June 1997, about her crush.
"I'm like, 'Judd, I love Brendan!' He's like, 'I know. Stop it,'" she said. "I started getting really weird around [Brendan] at work, you know, because he's like naked all day long."
Fraser shocked Mann by walking out on stage and presenting her with a bouquet of flowers. The pair sat down with Philipps, where Mann recalled a scene in George of the Jungle where Ursula voices her love for George.
"I would start to well up because I did love him!" the actress said.
Apatow, who is parent to daughters Maude and Iris with Mann, reacted to Mann and Fraser's reunion in a tweet Wednesday.
"I have mixed feelings about Leslie declaring her love for Brendan. I get it. But it does hurt. @BusyTonightTV @BusyPhilipps," the director wrote.
Apatow said Thursday the reunion was "a great segment."