Career
Lehtonen was drafted by Boston Bruins in the 3rd round (83rd selection) of 2005 National Hockey League Entry Draft. Mikko was drafted from Finnish team, Espoo Blues of the Master of Science-liiga. On May 31, 2007, the Boston Bruins signed Lehtonen to an entry-level contract.
Lehtonen made his North American debut in the 2008-2009 season with Boston"s American Hockey League affiliate, the Providence Bruins.
Lehtonen made his National Hockey League debut in the Bruins penultimate regular season game in a 6-1 defeat to the Buffalo Sabres on April 11, 2009. While with Skellefteå on February 28, 2011, Lehtonen"s National Hockey League rights were traded by the Bruins along with Jeff Penner to the Minnesota Wild in exchange for Anton Khudobin.
On May 4, 2011, Lehtonen was announced as the new player of KHL"s Severstal Cherepovets.