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.