The actors, both 31, are thought to be marrying in Leslie's 12th century family castle in Scotland on Saturday.
A date of June 23 was previously posted at Aberdeenshire's Huntly registration office.
The couple met while filming the fantasy saga in 2012 and played on-screen lovers Jon Snow and Ygritte.
They announced their engagement in the Times newspaper last year and are expected to marry at Wardhill Castle in Aberdeenshire.
Two years after they met, Leslie left Game Of Thrones, while Harington has become one of the show's most popular stars and has appeared in every episode.
Speaking to L'Uomo Vogue, Harington said it was "easy" to fall in love with Leslie and his best memory of the show was meeting his future partner while filming in Iceland in 2012.
"The country is beautiful ... the Northern Lights are magical ... it was there that I fell in love," he said.
"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."
The Good Fight actress Leslie said she had been too busy to plan the wedding, telling Town And Country magazine: "I haven't tackled it. There's just too much to do."
