Born in Malmö, Wilhelmsson started his career with Mjällby AIF in 1997, playing in three second division seasons with the club. He then spent as many years in neighbouring Norway with Stabæk Fotball.

Gaining his first cap in 2001, Wilhelmsson represented Sweden at UEFA Euro 2004, Euro 2008, Euro 2012 and the 2006 FIFA World Cup, featuring in every one of the national team’s four games in the latter tournament and starting in two.

In the 2003 summer Wilhelmsson joined R.S.C. Anderlecht in Belgium, being an undisputed starter during his spell and scoring nine top division goals combined in the capital side’s 2004 and 2006 league conquests.

In 2006, Wilhelmsson moved to French team FC Nantes. However, on 10 January of the following year, he joined A.S. Roma on loan until the end of the season, when the Italians had the option to buy for €3 million. He made his Serie A debut on 14 January against FC Messina,

On 4 September 2006 he was one of three international players sent home for breaking a curfew during a night out, the other two being Olof Mellberg and Zlatan Ibrahimović. In March 2011, he was involved in a training ground scuffle with the latter.

He was again loaned by Nantes, this time moving to Spain and Deportivo de La Coruña on 30 January 2008. He played his first La Liga match against Getafe CF, coming on as a substitute in a 1–1 home draw, and made his first start two rounds later, in a home fixture match against RCD Espanyol (2–0 home win).

Wilhelmsson scored his first goal for Depor on 1 March 2008, in a 2–1 home win against Sevilla FC, and finished the campaign firmly established in the side’s starting XI – he would start in 13 of his league appearances – as the Galicians eventually finished ninth and qualified to the UEFA Intertoto Cup.

In 2008, Wilhelmsson signed a three-year contract with Saudi club Al-Hilal FC for about €9 million. He helped the club win the league title in his second year, scoring nine goals and making ten assists in 20 appearances; for his contributions he received the Player of the Season award, going on to win four additional major titles during his tenure, including two consecutive Saudi Crown Prince Cup where he scored in both finals.

Wilhelmsson married Russian-born model Oksana Andersson in May 2010, in Las Vegas. Aside from Swedish, he also speaks fluent English.

Wilhelmsson scored his first brace for Sweden in a Euro 2012 qualifier against San Marino on 6 September 2011 (5–0 away win), for his seventh and eighth international goals. He was also selected for the finals in Poland and Ukraine by manager Erik Hamren, collecting three bench appearances in an eventual group stage exit.

Wilhelmsson was announced as a LA Galaxy player on 5 September 2012. He scored in his Major League Soccer debut nine days later, in a 2–0 win against the Colorado Rapids.

On 14 January 2013, Wilhelmsson signed a one-and-a-half-year deal with Baniyas SC. After a spell with former club Mjällby, the 35-year-old officially announced his retirement. However, in August 2015, he came out of retirement to help the latter team retain their status in the Superettan, which was eventually not achieved.