[8] He started practicing handball at the age of 8 with Valur's junior teams but after breaking his right arm in a horse riding accident, which affected his ability to throw the ball with power, he turned to basketball at the behest of former national team player Sigurður Helgason.

[9] Due to his size and skills, he was selected to the Icelandic junior national teams where he was noticed by Marv Harshman, the head coach at the University of Washington.

Harshman arranged for Pétur to move to Seattle and join Mercer Island High School at the age of 16.

[18] After his third college season, Pétur signed with Club Atlético River Plate in Argentina during the summer of 1980.

"[21] After the season ended in Argentina in November, Pétur briefly joined KR[22] and played with the team in an unofficial continental tournament in England.

[27] Pétur was selected in the third round of the 1981 NBA draft by the Portland Trail Blazers, for whom he played during the 1981–82 season.

[27] In August 1983, the Blazers traded his rights to the Detroit Pistons in exchange for a 1984 third-round draft pick.

[45] In 1986, Pétur was set to play for the CBA's Kansas City Sizzlers but was denied a work permit by the U.S. Immigration and Naturalization Service.

[51][52] Prior to the season, during training camp, he suffered a back injury[53] that was later diagnosed as a slipped disc and eventually required surgery.

[54] While recovering, he was traded to the San Antonio Spurs[55] along with Frank Brickowski,[56] two draft picks and an undisclosed amount of money in exchange for Mychal Thompson on 13 February 1987.

[58] After being one of eight Spurs players protected in the 1988 NBA expansion draft,[59] Pétur started the first game of the 1988–89 season, a 122–107 victory against his former team Lakers, where he scored 7 points in 21 minutes.

[60] He was again in the starting lineup during the second game of the season but suffered a knee injury in the fourth quarter that required surgery.

[68] Pétur played with the Sioux Falls Skyforce from January[69] to March 1990, where he averaged 5.8 points and 6.0 rebounds per game in 22 appearances.

[70] In October 1990, Pétur returned to the Úrvalsdeild karla for the first time in six years and signed with Tindastóll[71] for a reported 2.3 million ISK salary for the season.

Due to Pétur's injury, Tindastóll's play faded in the second half of the season[74] and the team lost 9 of its last 12 games, ending with a 15–11 record and finishing fifth, two victories out of the playoffs.

[27] After contemplating retirement due to a back injury,[83] Pétur signed with a recently promoted Breiðablik in August 1992.

In January 1993, Pétur announced that he intended to join Valur for the rest of the season and end his career where it started.

[6] After failing to receive a playing contract in the United States, Pétur signed as a player-coach for ÍR in January 1984.

[90] In August 1993, prior to the start of the season, he resigned from his post for personal reasons and moved to the United States.

[91][92] Pétur returned to Valur in June 2000 and signed a three-year contract to coach the joint team of Valur/Fjölnir in the Úrvalsdeild karla.

After a poor start, the team finished strong and made the playoffs, where it was knocked out in the semi-finals by eventual champions Keflavík.