Career
Koskinen was drafted 31st overall by the New York Islanders in the 2009 National Hockey League Entry Draft after playing for the Espoo Blues in the Finnish Master of Science-liiga. He was the first goalie taken in the draft as well as the first Finnish player selected. Prior to the draft he was the second ranked European goaltender by National Hockey League Central Scouting Bureau.
On July 13, 2009, Koskinen signed a three-year entry level contract with the New York Islanders.
In the 2009-2010 season (his first in North America) Koskinen was assigned to the Islanders affiliate the Bridgeport Sound Tigers to start the year. After only two appearances with the Sound Tigers, Mikko suffered a torn Labrum in his hip which required surgery and ruled him out for the majority of the season.
On March 19, 2010, after four months of rehab, Koskinen returned to health and was reassigned to secondary Islanders affiliate, the Utah Grizzlies for the remainder of the year. Undefeated in six starts with the Grizzlies he helped the Grizzlies reach the conference semi-finals before returning to the Sound Tigers first round playoff series on April 18, 2010.
Koskinen made his National Hockey League debut on February 8, 2011 against the Toronto Maple Leafs.
The Islanders were defeated 5-3. On November 12, 2011, Koskinen returned to Finland, signing with KalPa.