The pair, who played a couple in Game of Thrones, have been dating since 2012 but only officially stepped out in public together at the 2016 Olivier Awards.
In a recent interview with Vogue Italia, Harington talked about how they fell for one another while filming Thrones in Iceland.
Speaking on The Tonight Show, Harington revealed he shared the secret with a cop who pulled him over for driving too fast.
"I was going over the speed limit and I feel the sirens go off behind me and this policeman comes up, pulls me over," Harington said.
"He said, 'look, there's two ways we can do this: You can either follow me back to the police station now and I book you in or you can tell me whether you live in the next series of Game of Thrones.'"
So Harington told him and the policeman's response was: "On your way, Lord Commander".