Willum led
Þróttur to Iceland's top league, the
Úrvalsdeild in 1997. The wait had been long as Þróttur last played at the top level in 1985. In 2000, he became the manager of
Haukar, and in 2002, he was appointed manager of
KR and led them to two successive
championships in 2002 and 2003. In 2004 KR finished without a title and Þórsson's contract was not renewed. He took charge of
Valur who had just been promoted to the Úrvalsdeild and finished the 2005 season in 2nd place after champions
FH. Valur won the 2005
Icelandic Cup with a 1–0 victory over
Fram. In 2006, Valur ended the season in 3rd place and in 2007 Willum managed
Valur to their first League title in 20 years and won Manager of the Season in
Landsbankadeildin. He left with mutual agreement on 1 July 2009. On 29 September 2009, after the season had finished for the year, he was appointed the new manager of
Keflavík. He is the only manager who has won every single league in Iceland (4 in total), 2nd and 3rd division with
Haukar,
1st division with
Þróttur and the
Premier League with
KR and
Valur. In November 2010, Willum was appointed as the first manager for the Icelandic
futsal team. ==Political career==