Fergus appears to have had strong connections in Fife, and it is possible that his father (if he was his father) Colbán was a Fifer.
A charter issued by Fergus appears to have survived.
The charter had a few witnesses with French names, presumably a phenomenon related to his Comyn connections.
Fergus had no male heirs, and married his only daughter Marjory to William Comyn, bringing Gaelic control of the Mormaership to an end.
On Fergus' death, Buchan became the first native mormaerdom to pass into the hands of a foreign family He died sometime before 1214, possibly much earlier.