His parents were Gissur Teitsson and Þórdís Þóroddsdóttir. After studying in Herford in Germany, he was made bishop of Iceland in 1056 by Athelbjart, archbishop of Bremen. He built up a see in his family homestead in Skálholt and founded a school. One of his students was Jón Ögmundarson (1052-1121), who later became the first bishop in Hólar. Ísleifur served as bishop for 24 years, right until his death. His wife was Dalla Þorvaldsdóttir and they had three sons: Þorvaldur, Teitur and Gissur, the latter taking over as bishop after his father's death in 1080. 
- Sauser, Ekkart (2001). Biographisch-Bibliographisches Kirchenlexikon (BBKL) (in German). Herzberg: Bautz. p. 704. ISBN 3-88309-086-7.