Marteinn's siblings included the sýslumenn Pétur [is] and Brandur Einarsson, known as Gleraugna-Pétur and Moldar-Brandur respectively, as well as a sister, Guðrún, the wife of the magistrate Daði Guðmundsson.
Marteinn spent nine years studying in England, where another of his sisters lived after marrying an Englishman, before returning to Iceland to work as a merchant in Grindavík.
Marteinn remained detained until after the Battle of Sauðafell in 1550, where Daði Guðmundsson defeated Jón Arason, ending organized Catholic resistance to the Reformation, at which time he formally took control of the bishopric of Skálholt.
Marteinn lived out his retirement in Álftanes á Mýrum [is] where he published a collection of psalms and was considered a good painter, although none of his paintings are known to survive.
Marteinn appears in Ólafur Gunnarsson's 2003 historical novel Öxin og jörðin (translated into English as The Ax and the Earth), which is set amid Jón Arason's struggle against the Icelandic Reformation.