The couple met on the hit HBO fantasy show in 2012.
They played on-screen lovers Jon Snow and Ygritte in the hugely popular series.
The pair will marry in Aberdeenshire on 23 June.
The date was posted at the Huntly registration office.
Leslie’s father, councillor Seb Leslie, owns a castle in his West Garioch constituency.
Harington has become one of the show’s biggest stars after appearing in every series of the popular TV drama, which has become renowned for killing off lead characters.
Leslie left the cast two years after they met after her character was killed off.
The Scottish BAFTA-winning actress first rose to fame as Gwen Dawson in the ITV drama series Downton Abbey.
Harington earlier told L’Uomo Vogue it was “easy” to fall in love with Leslie.
He said his best ever memory of the show were the three weeks in Iceland when they filmed the second season in 2012.
“Because the country is beautiful, because the Northern Lights are magical and because it was there that I fell in love,” he told the magazine.
“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.”