Aspelin began his professional career in 1998. During the same year he achieved his best finish on the tennis world rankings as a single player. The ATP led him on August 17, 1998 to rank 436
Greater success he achieved solely as a doubles player. His first ATP title after a series of Challenger and Futures successes he won at the side of his compatriot Johan Landsberg in 2000 in Marseille. In the same year the Swedish double qualified for the Doubles Championships in Bangalore. Until the 2007 season, the right-handed access with Massimo Bertolini and Todd Perry five further double successes, including the International Series Gold tournament in Memphis on the side of Perry.
The year 2007 was the most successful year of Sweden when he was successful in conjunction with Austrian Julian Knowle in four tournaments. The biggest triumph of his career, he reached again on the side of Knowle, by winning the doubles competition at the U.S. Open 2007. The set only as number 10 pair defeating in the final the Czechs Lukáš Dlouhy and Pavel Vízner 7:5 and 6:4. This season, he again reached the Double Championships at the end and won $ 432,685 in prize money, which is his personal record. On 5 November 2007 Aspelin climbed to number 8 in the double world ranking, his best-ever placing.
In the 2009 season with Paul Hanley he succeeded his next triumph by winning the German Open in Hamburg. He also reached the final in Casablanca Hanley and Wesley Moodie the final of the Masters tournament in Madrid. His last title he won in 2010 at the ATP tournament in Dubai. His partner was also Paul Hanley. On July 17, 2011 Aspelin said after the final defeat at the tournament in Båstad his retirement from professional tennis. He won a total of twelve ATP tournaments, including the U.S. Open. He also reached the final of another 21 tournaments.
In the years 2001-2011 Simon Aspelin participated for Sweden at the Davis Cup competition; his partner was until 2007 Jonas Björkman. Of the seven matches ( including against Russia, France, India and Argentina) could this pair only two decide for themselves. As of February 2008 Aspelin played alongside Robert Lindstedt. They won four of their five common games. Aspelin also completed two single matches, but was defeated both times.