[3] Pétursson was born in Blönduhlíð, the son of Péturs Péturssonar, a professor in Víðivöllur and his second wife, Thóra Brynjólfsdóttir.

Pétur and his brother Brynjólfur were educated by the Reverend Einar Thorlacius in Goðdölar and later in Eyjafjörður.

He was appointed bishop of Iceland in 1866 and served his office for 23 years.

His first wife was Anna Sigríður Aradóttir from Flugumýri who died in 1839 after a short marriage.

His second wife was Sigríður Bogadóttir, the daughter of Boga Benediktsson and Jarðrúður Jónsdóttir.