The actress starred in the first series of Downton Abbey as Gwen, who Lady Sybil helped to advance from a housemaid to a secretary.
In an interview with Access Hollywood, Leslie revealed that she's not opposed to reprising the role of Gwen in the future.
"There is no way I would ever say, 'No'," the star declared. "I had the most fantastic time filming Downton. I really did, so I would love to go back, but... I was always told that I would only ever be in the first series."
Leslie added: "I always knew that my character would have a happy ending at the end of it all. [Leaving Downton Abbey] didn't come as a shock... But it was fantastic. I loved it."
She went on to credit Downton Abbey with helping her land her current job as Ygritte on Game of Thrones.
"I definitely think it opened doors for me and what was so fantastic about Downton being showed over in the States, was that it helped get me noticed," Leslie explained.
"I know that [Game of Thrones executive producer] David Benioff actually mentioned it to me, I think it was on my first kind of audition going for Ygritte.
"He was like, 'We've seen you in Downton Abbey and we like the whole, like, Northern accents thing'. And I [thought], 'Oh! Great! Well, obviously that was working in my favour'. So, yeah... I definitely think it helped me, which is lovely."
Downton Abbey returns for a third series this summer on ITV in the UK and PBS in the US.