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Fergus was supposed to have been one of the seven Earls of Scotland who were displeased at Malcolm IV's alliance with Henry II at Toulouse and wanted to take him from the throne at the assembly at Perth in 1160. He made a grant of a mark of silver per year to the abbey of Aberbrothwick founded by King William.

Fergus of Buchan was the last native Gaelic Mormaer of Buchan, and only the third to be known by name as Mormaer. Fergus appears to have had strong connections in Fife, and it is possible that his grandfather (if he was his grandfather) Colbán was a Fifer. A charter issued by Fergus appears to have survived. The charter is a feudal charter granting lands to a subordinate. The charter had a few witnesses with French names, presumably a phenomenon related to the 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.

Ref:

The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct or Dormant, G.E. Cokayne; with Vicary Gibbs, H.A. Doubleday, Geoffrey H. White, Duncan Warrand and Lord Howard de Walden, ed., new ed., Alan Sutton Publishing, Gloucester, U.K., 2000- vol. II, p. 374
Lost Kingdoms: Celtic Scotland in the Middle Ages- John L. Roberts, Edinburgh, 1997- pp. 55-6
Buchan in the 13th century - Alan Young, in Alexander Grant & Keith J. Stringer (eds.) Medieval Scotland: Crown, Lordship and Community Essays Presented to G.W.S Barrow- Edinburgh, 1993
"The Scottish Nation"- William Anderson, A. Fullarton & Co., Edinburgh, 1880