3 times Mjolnir has been broken.
1. Thor vs. The Destroyer
From Journey Into Mystery #118
A pissed off Destroyer fires a blast that slices a large chunk off Mjolnir.
2. The Avengers vs. Molecule Man
From Avengers #215
The Molecule Man captures the Avengers and Silver Surfer and uses his powers to simultaneously destroy Captain America’s shield, Iron Man’s Armor, Silver Surfers board and Mjolnir.
3. Thor vs. Bor
From Thor vol. 3 #601
Under Loki’s spell, a resurrected Bor and Thor do battle. Thor delivers the final mighty blow with such force that part of Mjolnir shatters.