Pétur helped Skallagrímur reach the 2006 Úrvalsdeild finals, scoring 21 points in the fourth and deciding game against defending champions Keflavík in the semi-finals.[2][3] In the finals, Skallagrímur lost 1-3 to Njarðvík.[4][5]
Pétur turned became a player-coach for Laugdælir in 2009.[6] In 2010 he led them to a perfect 16-0 season, while also going 5-0 in the playoffs,[7] the Division II championship and promotion to Division I. Despite not fielding a foreign player, his squad managed to stave off relegation during the 2010–2011 Division I season and reach the final eight of the Icelandic Basketball Cup.[8] After the season Pétur was hired as the head coach of KFÍ. In his first season he led KFÍ's men's to the Division I championship and promotion to the Úrvalsdeild karla, while also reaching the final four in the Icelandic Basketball Cup where they lost to eventual champions Keflavík.[9] After a 10th-place finish in the 2011-2012 season, where the club staved off relegation with a victory in their last game, Pétur left KFÍ when his contract was not renewed.[10][11]
In May 2015, Pétur was hired as an assistant coach to Stjarnan men's team.[12] In 2016, Stjarnan hired him as the head coach of the women's team[13] On 13 February 2019, Pétur Már guided Stjarnan women's team to the Icelandic Cup finals for the first time in the club's history, after a 103-82 victory against Breiðablik in the semi-finals.[14]
In June 2019, the board of Stjarnan announced that it would withdraw the women's team from the Úrvalsdeild kvenna and register it for the second-tier 1. deild kvenna instead due to several key players leaving the team. Shortly later, Pétur left the team and signed with Vestri.[15][16]
In June 2021, Pétur guided Vestri back to the Úrvalsdeild after the team defeated Hamar in the 1. deild promotion finals.[17] In July 2021 he left his post as head coach of Vestri's women's team to fully focus on coaching the men's team.[18]