The Batman: The Dark Knight Rises star was heavily made-up for the role, in which he plays a younger version of a time-travelling Bruce Willis.
The film is set in a world where illegal time travel is used to send enemies of the mob 30 years back in time to be killed by a specialist assassin called a 'looper'.
The film centres around looper Joe (Willis), who is sent back to be killed by his younger self (Gordon-Levitt).
Gordon-Levitt has praised the prostheses that he wore during filming, claiming that his own mother did not recognise him.
"They did such amazing work," he said of the two-and-a-half hour make-up process. "I would look in the mirror and think that was my face!
"It got to the point that on weekends, when I had my normal face, it was weird to me. To have a different face, it changes everything. That's what I love about acting."
Emily Blunt, Paul Dano, Noah Segan, Piper Perabo and Jeff Daniels will also star in the movie.
Looper will open in the US and UK on September 28.