2008-09: Mike Halmo played 62 games for Owen Sound in his first OHL season. He scored 5 goals with 3 assists and was -9 with 90 penalty minutes. The Attack finished fourth in the Midwest Division and faced Memorial Cup champion Windsor in a first round playoff series. Halmo had 1 assist and was -5 with 2 penalty minutes in four playoff games.
2009-10: Halmo skated in 60 games for Owen Sound and led the Attack with 121 penalty minutes in his second season. He scored 11 goals with 18 assists and was -4. Owen Sound missed the playoffs after finishing fifth in the Midwest Division.
2010-11: Halmo was one of seven 20-goal scorers for the OHL champion Attack in his third season with Owen Sound. He also had 23 assists and was +10 with 121 penalty minutes in 59 games. Owen Sound was first in the Midwest Division and defeated Mississauga St. Michael's in the OHL finals. Halmo scored 5 goals with 10 assists and was +8 with 36 penalty minutes in 22 playoff games. He scored 1 goal with 4 assists and was +2 with 6 penalty minutes in four Memorial Cup games.
2011-12: Halmo signed a three-year, entry-level contract with the Islanders as a free agent in March 2012 and made his pro debut with AHL Bridgeport after leading the OHL's Owen Sound Attack in scoring and penalty minutes in an over-age season. He scored1 goal and was plus-one with one fighting major in five AHL games. Halmo scored 40 goals with 45 assists and was plus-24 with 162 penalty minutes in 66 games for Owen Sound. He received a 10-game suspension at the end of the season for a hit on Edmonton prospect Nail Yakupov; missing the last three regular season games and the Attack's playoff series with Kitchener.
2012-13: Halmo played in 46 AHL games for Bridgeport as a 21-year-old in his first pro season. He scored 5 goals with 9 assists and was +2 with 46 penalty minutes. The Sound Tigers missed the AHL playoffs after finishing third in the Northeast Division.
Halmo began the 2013-14 season with the AHL's Bridgeport Sound Tigers.