As a child of the previous mormaer, he was entitled to succeed his father through primogeniture, but not to lead his kin-group, Clann MacDuib.
Like previous mormaers of Fife, Duncan II was appointed Justiciar of Scotia (i.e. Scotland North of the Forth).
Donnchad's minority also meant that Ferchar, Mormaer of Strathearn, took supreme place as head of the Gaelic nobility and guide for the boy-king Malcolm IV.
Duncan's person was required to be a hostage following the defeat of William the Lion and the Treaty of Falaise, although in fact he certainly sent someone else in his place (Barrow, 2003, 106).
In 1152, on the death of Scottish king David I's son Henry of Scotland, Duncan I had escorted Malcolm IV, introducing him as the royal heir.