Harington, 30, decided to propose to Rose following their house move earlier this year, which helped to convince him that his long-term future was with the Scottish star.
The insider told The Sun newspaper: "After moving in together in January he felt it was the right time to take the next step."
Harington and Leslie have both made a conscious decision to keep their relationship low-key, with the duo only making their debut as a couple last year.
Despite this, Harington has admitted playing Leslie's on-screen love interest helped their relationship to develop.
The actor, who stars in the role of Jon Snow in Game of Thrones, previously shared: "If you're already attracted to someone, and then they play your love interest in the show, it becomes very easy to fall in love."
Earlier this year, it was reported that the couple splashed out as much as £1.75 million on their new home in East Anglia, England.
Harington and Leslie - whose Game of Thrones character, Ygritte, left the show in 2014 - moved into a seven-bed, 15th century countryside pad and their arrival was warmly welcomed by the locals.
One villager said at the time: "How thrilling - everyone is talking about the glamorous couple from the Game of Thrones moving in.
"There are quite a few celebs round here including rock stars and well-known TV people - so no-one makes much of a fuss and locals are very respectful of their privacy."
- Bang! Showbiz
