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Lehkosuo played most of his career at HJK Helsinki. He was not a very prominent youth player and made his top division debut in 1993 after spending three years with lower division club Vantaan Pallo. He did not make his breakthrough into the HJK senior squad until 1995, when he made 24 league appearances and scored one goal. In the previous season he had even played one league match for FF Jaro.
In the following seasons Lehkosuo received more and more responsibilities and became one of the best midfielders in the Veikkausliiga. In the end of 1990s he formed the successful midfield trio with Aki Riihilahti and Jarkko Wiss with whom he won the league title in 1997 and worked their way to the group stage of UEFA Champions League in the next season.
In the winter of 1998 Lehkosuo was signed on loan by Italian Serie A side Perugia. He only played eleven games in Italy and returned to HJK for the next Veikkausliiga season. In 1999 and 2000 Lehkosuo was made the captain of the HJK squad and he played in a more attacking role. However, in August 2000 Lehkosuo injured his knee ligament. The knee didn't fully recover and Lehkosuo was forced to end his playing career in 2002 after playing 190 matches in Veikkausliiga, of which he played 189 for HJK.
Lehkosuo played 17 matches for the Finnish national team and scored one goal.
Lehkosuo started his managerial career with HJK youth teams quickly after retiring as a player. In 2005 he was appointed as assistant manager of Ville Lyytikäinen of Ykkönen (the second tier) side FC Honka. After Lyytikäinen was sacked, Lehkosuo became head coach. He guided the team to the top place of Ykkönen. In the first season in the Veikkausliiga Lehkosuo guided his team to the fourth place. In 2007 Honka was again fourth. In 2008 Lehkosuo guided Honka to second place and to the UEFA Cup qualifications for the next season. In 2009 Honka was again second in the league table.
Lehkosuo graduated as a Master of Science in Technology from Helsinki University of Technology in 2003.