Lehtonen was drafted by Boston Bruins in the 3rd round (83rd selection) of 2005 NHL Entry Draft. Mikko was drafted from Finnish team, Espoo Blues of the SM-liiga. On May 31, 2007, the Boston Bruins signed Lehtonen to an entry-level contract. Lehtonen made his North American debut in the 2008–09 season with Boston's affiliate, the Providence Bruins.