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 the 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. A sponsorship deal with a local radio station meant that Lehkuosuo wore the unusual shirt number 96.2 for a period at HJK Helsinki. This deal ended when he was forbidden to wear the number in UEFA Champions League matches. Lehkuosuo captained HJK in the group stage of the Champions League that season, helping HJK become the first ever Finnish team to earn that distinction. 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 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 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 managed FC Honka until February 2014. On 29 April2014, Lehkosuo was appointed as the manager of HJK as Sixten Boström was sacked. He led HJK into the Europa League group stages in 2014 with a 5-4 aggregate victory over SK Rapid Wien in the play-off round. He left the club in May 2019.
