The actress could not work in the acting industry with her birth name Emily Stone, as someone with the same name was already registered with the Screen Actors Guild.
Stone originally chose a different name but had difficulty while working on TV series Malcolm in the Middle.
The 23-year-old told Interview: "I was 16 at the time, and I hadn't done anything yet, so I was like, 'I can be whatever I want to be. What name do I want to have?' So I went though a list of names and picked Riley."
While on the set of the TV show, Stone repeatedly ignored staff by accident when they tried to address her with the name Riley.
She said: "Everyone kept on yelling, 'Riley!' and I didn't know who they were talking to."
She then decided to choose her nickname as her stage name, adding: "I had to change my professional name to Emma, which my mum always called me."