Not entirely. The movie takes the real story of Arne Johnson and the Warrens' investigation. But in real life, things are not as cut - and - dried as shown in the movie. The idea of demonic possession is very hard to prove. In the movie, they make it seem more certain, with all the spooky events and the Warrens' confident declarations. In reality, there was a lot of doubt and scientific scrutiny that the movie doesn't fully represent.
It's based on a real story to an extent. The case of Arne Johnson is real, and the fact that he claimed demonic influence is true. But the movie version of 'The Conjuring: The Devil Made Me Do It' has fictionalized elements. It heightens the horror and the supernatural aspects. For example, the movie might show more intense paranormal activities during the investigation than what actually happened. So it's a blend of truth and Hollywood - style exaggeration.