What was on his chest?

He first burst on the scene in 1942 as the dynamic fusion between Superman and Mickey Mouse, fuelled by the hype for both at the time. In fact he was originally named Super Mouse, but that was a little too close to the Superman brand for comfort.

Many remember his bright caped costume, but what was written on his chest?

If you thought it was a large red "M" you may be experiencing an MMDE, because all references today show it was blank.

In a strange quirk of fate, in 2005 he was again embroiled in a trademark dispute, but this time with the Apple corp when they launched the "Apple Mighty Mouse". He won that epic battle, and they had to change it to the "Apple Mouse".

Origin

While still called Super Mouse, the origin of his powers is shown in the first episode, "The Mouse of Tomorrow", aired in 1942. He escapes some cats by running into a Supermarket, where he eats Super Cheese and is transformed into Super Mouse. He's known for his signature tune, "Here I come to save the day", which is the one Andy Kaufman used in his famous routine. There was a strong attempt to push Might Mouse in the theatres but the wasn't successful, only when moving to television did the real recognition arrive.

This Mandela Effect is quite widespread. For some reason, many people picture the "M" on his chest unprompted, especially when asked to draw him. One explanation if the sheer number of characters with something there, for example SuperMan, BatMan etc, and with Mighty Mouse not being anything like as popular as many other characters, so not fully remembered, the addition of the "M" may just be something automatically added when the memory is retrieved.