The 30-year-old British actor gushed about Leslie in an interview with news.com.au while discussing the HBO show's impending end.
"'Thrones' has been with me throughout all of my 20s. It's been a journey, a strange journey," Harington, who plays Jon Snow, confessed.
"It's remarkable having come into a show not having done any previous work in TV or film, and it becoming this crazy, big cultural phenomenon."
"I met my girlfriend on the show and that has been the greatest gift of all," he said.
Leslie portrayed Jon Snow's love interest, the wildling Ygritte, in Seasons 2, 3 and 4 of "Game of Thrones."
Harington said he also developed close friendships with several of his co-stars, whom he hopes to stay in touch with after the show ends.
"I certainly hope one of the people who will remain in my life is Rose," the actor jokingly said.
"I'll stay in touch with Alfie Allen (Theon Greyjoy), and I am still friends with Richard Madden (Robb Stark), John Bradley (Samwell Tarly) and Liam Cunningham (Davos Seaworth); we all meet up and have dinner," he added.
Harington and Leslie were first linked in 2012, and made their red carpet debut as a couple at the 2016 Olivier Awards.
The actor said in the June/July issue of Esquire that he and Leslie have moved in together.
"It's as much her relationship as it is mine and I can't speak for both of us," he told the magazine. "But yeah, we are very, very happy. So that's what I'll say about that."
Harington will return as Jon Snow in "Game of Thrones" Season 7, which premieres Sunday. The series will come to a final close with Season 8.
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