It was formed in 1152 out of the diocese of Oslo, when Arnold, Bishop of Garðar, Greenland (1124–1152), was appointed first Bishop of Hamar. He began to build the now ruined cathedral of Christ Church, which was completed about the time of Bishop Paul (1232–1252).
The last Catholic bishop, Mogens Lauritssøn (1513–1537), was taken prisoner in his castle at Hamar by Truid Ulfstand, a Danish noble, and sent to Antvorskov in Denmark, where he was held until his death in 1542. There were at Hamar a cathedral chapter with ten canons, a school, a Dominican Priory of St. Olaf, and a monastery of the Canons Regular of St. Anthony of Vienne.
(in Danish) J. M. Hansen, Hamar og dets Biskopper (Hamar and Its Bishops), Hamar: Th. A. Hansens Bogtrykkeri, 1866.
(in Norwegian) Chr[istian]. Ramseth, Hamars Bys Historie. Til 50 aars jubilæt 21 Mars 1899 (History of the City of Hamar : 50th Anniversary, 21 March 1899), Hamar: L. Larsen, Axel Magnussen and H. A. Samuelsen, 1899.
(in Norwegian)Christian C[hristoph]. A[ndreas]. Lange, De norske Klostres Historie i Middelaldern, Anden omarbeidede Udgave (The History of the Norwegian Monasteries in the Middle Ages, Second Revised Edition), Christiania [now Oslo]: Chr. Tønsbergs Forlag, 1856, pages 374-77 and 389–391; Historisk Tidsskrift (Historical Journal), 3rd series, Volume I, Christiania, 1890, pages 113–140, 244–269, 277–334; Volume III, Christiania, 1895, pages 379–411.