[4] He was a key player in the ÍR team that won the Icelandic championship for five straight seasons, from 1960 to 1964.
[2] In 1965, Þorsteinn joined SISU BK in the Danish Basketball League and helped the club to the national championship in 1966 and 1967.
After the season he moved back to Denmark and rejoined SISU,[2] winning the Danish championship in 1972 and 1973.
[4] Þorsteinn retired after winning the 1977 Icelandic championship with ÍR,[4][6] but later appeared in three games during the 1979-1980 season before quitting for good.
[7] Þorsteinn, at the age of 16, played in the Iceland's first official national team game,[8] against Denmark.