Despite the injuries and two years of college eligibility remaining, the North Stars signed the talent but diminutive center. Thyer ended up dropping out of college to turn pro immediately, as the Stars felt his development as a hockey player would improve if he took his game to the next level. Thyer joined the Stars IHL affiliate, the Kalamazoo Wings.
Mario showed flashes of brilliance in his first professional season in 1989-90. In 68 games the speedy center netted 19 goals and added 42 assists for 61 points while taking only 6 minor penalties. He also got what proved to be his only shot at the NHL as he went scoreless in 5 games with Minnesota.
Thyer had NHL speed and NHL puck skills, but lacked NHL size. He weighed only 170lbs, and a center ice its rare to find such a puny pivot. His lack of size would be his biggest obstacle to playing in the NHL..
Thyer returned to Kalamazoo for the 1990-91 season and scored 15 goals, 51 assists and 66 points. 1991-92 was an injury plagued season for Thyer, who did scored 17 goals and 45 points in 46 games with Kalamazoo before he was traded on March 10, 1992. The Stars felt they had enough small and soft centers with names like Dave Gagner, Neal Broten and Mike Modano already on the big club and were looking for a hulking centerman for their playoff run. The Stars traded Thyer to the New York Rangers in exchange for big Mark Janssens.
However Thyer's stay with the Rangers was short. He finished the 1991-92 season with the Rangers farm team before he was traded back to Minnesota on July 16, 1992. Thyer was unhappy because he knew there was little chance he could crack the big club in Minnesota. He ended up playing with the IHL's Cincinnati Cyclones that year, but struggled through a 13 goal, 49 point season in 77 games.
Thyer only played in 3 games in 1993-94 before he decided to hang up the blades. He knew he was never likely to make the NHL and decided to get on with his life.