Mr. M was Mario's evil alter-ego in Into the Fourth Dimension. He was created when Mario was kidnapped by Dowser and was brought to Dimentio to be brainwashed. He later invaded 4D Tower and battled Luigi, his brother who once was brainwashed into becoming Mr. L, and lost. He later stalked the heroes and then asked Dimentio something about his mind control powers. Then he, Bob-omberman and Dowser all kidnapped MarioMario54321 and Dimentio turned him into Mr. Mx2. He is Mr. L's brother
Mr. M's game was later ended by Dimentio after he betrayed Dimentio from losing full control of himself, and the real Mario was sent to the Game Over Zone along with MM54321.