Mirren, 70, gave Colbert, 51, an extra-special greeting when she walked out onstage and planted a kiss on the host's lips, leaving Colbert completely flustered.
"I have been dreaming of doing that for about 15 years, so I just grabbed my chance," said Mirren when they sat down.
"That is one of the nicest greetings any guest has ever given me," responded Colbert after taking a minute to stop blushing and collect himself.
And even after chatting with the actress about her status as a dame and her past working at a carnival, Colbert couldn't stop thinking about the kiss.
As he thanked Mirren for coming on the show, he grabbed both her hands and slowly started to pull her in for another kiss – and she certainly didn't seem to mind.
After a second sweet kiss, they both pulled away looking longingly in each other's eyes.
The bar has officially been set for all other late-night guests.