Let's let Jacques Plante's Hockey Hall of Fame biography tell this one:
Throughout his career he was plagued with recurring asthma and after missing 13 games due to a sinusitis operation, Plante began wearing a mask in practices in 1956. Coach Toe Blake endorsed the move cautiously because it kept his goalie healthy and happy, but he warned Plante that a mask wasn't permitted during games. However, during a Montreal versus New York game the night of November 2, 1959, Plante was hit in the face by a shot. He went off to the dressing room for stitches and when he returned he was wearing a mask. Blake was livid, but he had no other goalie to call upon and Plante refused to return to the goal unless he kept the mask. Blake agreed on condition that Plante discard the mask when the cut had healed. In the ensuing days Plante refused, and as the team continued to win, Blake became less obstinate. The Montreal record stretched into an 18-game unbeaten streak with Plante protected and the mask was in the NHL for good.
This photo was taken in November 1959, a few days after Plante's nose was broken and shredded. In the background, a pair of prototypes. It took 15 years, but in 1974 Pittsburgh's Andy Brown played his last NHL game, and no NHL goaltender would go maskless again. (Brown was a tough dude. He set a record for penalty minutes by a goalie with 60 in that 73-74 season.)
Here's the masked Plante in action: