On 16 August 1987, Sigursteinn made his first-team debut for KR, coming on as a second-half substitute for Gunnar Skúlason in the 0–1 defeat to Keflavík.
[6] He was selected in the starting line-up for the first game of the season against Leiftur on 15 May 1988, but was forced to leave the pitch in the tenth minute after sustaining an injury in a clash with an opposition player.
However, the season ended in disappointment for ÍA as they won only 3 of their 18 league fixtures and finished bottom of the division and were thereby relegated to the second tier of Icelandic football.
Sigursteinn played 17 matches and scored 5 goals during the 1991 season as the team regained their place in the top division after a one-year absence.
Over the next four seasons he remained a regular starter as the side won five consecutive league championships, becoming the first club to accomplish the feat since Fram more than 80 years earlier.
[9] That year, he was offered a chance to turn professional by moving to Swedish outfit Örgryte IS, but he declined and went on to stay with ÍA until the end of the 1998 season, making well over 160 appearances in all competitions for the club.
He played a total of 20 league and cup matches and scored once, netting the team's fourth goal in the 5–1 win against Valur on 27 May 1999.
In November of the same year, he joined English Football League Second Division side Stoke City on loan,[11] along with manager Guðjón Þórðarson and a number of other Icelandic players.
[2] Sigursteinn made his debut for Stoke in the 4–0 away win over Wycombe Wanderers on 23 November 1999, although he was substituted for compatriot Einar Daníelsson after suffering a dislocated shoulder.
[11] He failed to firmly establish his first-team place during his time in England, starting only four league matches for Stoke and making a further four substitute appearances.
During his second spell with KR, Sigursteinn played 72 league matches,[5] in addition to several cup appearances, including 10 games in European competitions.
[5] Sigursteinn received his first call-up to the Iceland national team for the friendly international against Tunisia on 17 October 1993, one of four uncapped players selected in the 16-man squad by manager Ásgeir Elíasson.
[15] Several of his ÍA team-mates also played for Iceland during this time, including Ólafur Þórðarson, Haraldur Ingólfsson and Sigurður Jónsson.
[15] On 15 August 1999, Sigursteinn was drafted into the Iceland squad for the friendly against the Faroe Islands after Sigurður Jónsson was forced to withdraw due to injury.
[19] In November 2004 he was hired as reserve team coach at his former club KR,[20] and appointed fellow former Iceland international Einar Daníelsson as his assistant.
[24] At the end of the 2008 season, Sigursteinn was appointed as manager of 1. deild karla side Leiknir Reykjavík on a three-year contract.
[25] He made several new signings ahead of the 2009 campaign; three players arrived from Leiknir's feeder club KB, while Kristján Páll Jónsson returned from a loan spell with Tindastóll.
A number of players left the club during the close season including Jakob Spangsberg, the top goalscorer in 2008, and first-team captain Vigfús Arnar Jósepsson.
[42] Sigursteinn's manager during his second spell with KR, Willum Þór Þórsson, paid tribute to his positive attitude as a player and praised his leadership abilities,[43] while his former ÍA and Iceland team-mate Ólafur Adolfsson described him as a popular man who always put the interests of the team above his own.