According to the Hockey News:
The 19-year-old goaltender agreed to a three-year deal with the Habs on Thursday after attending training camp on a tryout. He's a member of the QMJHL's Saint John Sea Dogs. Mayer is 1-1 in two games this season after posting a 16-11-0 record last year.
The Czech-born goalie has dual citizenship with Switzerland and represented that country at last year's world junior championship.
Mayer attended Montreal's development camp in July before earning an invitation to rookie camp and training camp.