After stints with Breiðablik, Tindastóll and St. Mary's High School in Canada, Sigtryggur had his breakout season with Skallagrímur during the 2014–2015 Úrvalsdeild karla season, averaging 16.7 points and 5.0 assists. Despite his performance, Skallagrímur was religated to 1. deild karla after the season. He had a standout season in the 1. deild karla, helping Skallagrímur achieve promotion back to the Úrvalsdeild after averaging 21.0 points, 5.7 rebounds and 5.0 assist, and was named as the leagues Domestic Player of the Year. He continued his stellar play during the 2016-2017 season, averaging 18.0 points and 5.3 assists, and was called up to the Icelandic national teamfor the first time after the season. He was though once again unable to help Skallagrímur avoid religation and signed with Tindastóll during the off season. On December 15, 2017, Sigtryggur was named to the All-First team of the first half of the 2017-2018 season On January 10, 2018, he scored 35 points in a victory against Haukar in the Icelandic Basketball Cup semi-finals. On January 13, 2018, he helped Tindastóll to its first major title when they beat KR in Icelandic Basketball Cup finals. On May 4, 2018, Sigtryggur was named to the Úrvalsdeild Karla Domestic All-First Team. On July 3, 2018, it was reported that Sigtryggur wanted to leave Tindastóll and join Grindavík, and that he considered his contract with the club not valid as the club did not send it to the Icelandic Basketball Federation within 30 days from its signing as the federations rules stipulated. The chairman of Tindastól in turn said that they viewed the contract as valid and that they expected Sigtryggur to report to the team for training camp before the season. The next day the clubs reported that they had reached an agreement and Sigtryggur signed with Grindavík later that day. In February 2020, he helped Grindavík to the Icelandic Cup Finals where it lost to Stjarnan despite leading all scorers with 23 points.
In July 2017, Sigtryggur was among the 24 players selected for the Icelandic national team training camp prior to EuroBasket 2017. He played his first national game on July 27, 2017, in a victory against Belgium. He made the cut to the final 15 players but was ultimately not named to the twelve man roster for Eurobasket. In November, 2017, Sigtryggur was named to the national team for the upcoming games in the 2019 FIBA Basketball World Cup qualification.
Personal life
Sigtryggur's father, Björn Sigtryggsson, played six seasons in the Úrvalsdeild karla with Tindastóll, Breiðablik and Valur. His uncle, Ómar Sigmarsson, played nine seasons in the Úrvalsdeild, eight of them with Tindastóll and one with Hamar.
