For her childhood starring role as Eli in the critically-acclaimed vampire romance Let the Right One In, she won a 2008 Online Film Critics Society Award, a 2009 Wonder Award, and a 2009 Fangoria Chainsaw Award.
Before winning the role of Eli, she went through three rounds of auditions, including an initial online casting submission.
She was cast in an Estonian film called The Arbiter and in a Swedish movie titled Tenderness.
Throughout her childhood in Falun, Sweden, she participated in local theatrical productions.
In Let the Right One In, her character's voice was dubbed by actress Elif Ceylan; the director of the film, Tomas Alfredson, wanted the young vampire's voice to be less high-pitched and young-sounding than Leandersson's natural voice was.