It is easy to gain the impression that Mim originated with the movie, as White published several different versions of his novel over the course of his life and the final version of the text does not mention her at all; but she is there to be found in earlier editions, magical duel with Merlin and all.
The Disney version of the character was adopted into the Donald Duck Universe where she sometimes teams up with Magica De Spell and/or The Beagle Boys. She sometimes appeared in the Mickey Mouse Universe where she teamed up with Black Pete on occasion and she even teamed up with The Phantom Blot at one point.