He succeeded his father in 1154 as Earl of Fife. "…Comite Duncano, Gillebrid comite de Anegus, M. comite de Ethoel, Gillecrist comite de Meneteth, Gillberto filio comitis Ferteth, Merlefwano Adam filio comitis de Anegus…" witnessed the charter dated 1164 under which "Malcolmus rex Scottorum" confirmed the foundation of Scone abbey. "Duncano comite de Fyfe…" subscribed the undated charter under which "Comes David frater regis Scottorum" founded Lindores Abbey. "Dunecan…comes de Fif" confirmed "terram…Gillecamstone", donated by "comes Dunecan pat meus", to North Berwick St Mary by undated charter witnessed by "Hela comitissa, Malcolmo filio comitis…Adam fratre comitis…". "William Masculus de Foules" donated "capellam de Foules" to the church of St Andrew’s by undated charter witnessed by "Comite Duncano, Adam fratre comitis…". "…Dunec, Malcolmo filio eio, com. Gilleb, Eth filio com. de Leueñ…" witnessed the undated charter, dated to 1193, under which "Dunecanus filius Gillebti filii Fergi" donated "totam terram de Moybothelbeg…[et] de Bethoc" to Melrose abbey. "Dunecanus comes de Fif" donated "ecclesiam de Cupre" to St Andrew’s priory by undated charter witnessed by "Hela comitissa, Adam fratre comitis…".
Justiciar of Scotland. "Davidem Ruffum de Forfar" donated "tota terra mea de Kinefe que fuit Ede" to Cupar abbey, for the souls of "Domini mei Willielmi et filii eius Alexandri", by undated charter witnessed by "Comite Duncano, Comite Gartneto, Comite Gilchristo de Anegus, Philippo Camerario, Villielmo Cumyn, David de Haya".
The Extracta ex Cronicis Scocie records the death in 1204 of "Duncanus comes de Fyff". 1
Generally referred to as Duncan MAC DUFF.