William of Mar, also known by the nameUilleam mac Dhonnchaidh (Anglicized as "William, Duncan's son"), was themormaer of Mar in medievalScotland from 1244 to 1276. His father wasDonnchadh of Mar.

Uilleam was responsible for the construction ofKildrummy Castle, the greatest castle to have been built in 13th-century northern Scotland. It is one of the few examples where anative Scottish magnate built a large-scalefortification, something normally practiced by the incomingNormans.

Uilleam, more than any of his predecessors, participated in Scottish and even British-wide politics, becoming a leading figure in the royal regime ofAlexander II, and the minority ofAlexander III. By 1244, Uilleam had married into the Comyn house, the fastest rising Scoto-Norman family in the Scottish kingdom. He married Elisabeth Comyn, the daughter ofWilliam Comyn, jure uxoris Earl of Buchan andMarjory, Countess of Buchan. The Comyn-Mar alliance helped fight off the ambitions of the Durwards, who were then in prime favor with the king.

Alan Durward used his descent from a daughter ofGille Críst to contest Uilleam's right to the Mormaerdom, but Uilleam successfully held off these claims. Uilleam and theComyn Earl ofMenteith then launched accusations of treason towards Alan while at the court ofHenry III of England atYork.

Uilleam engaged in supplementing his power on a nationwide basis. He held the post ofSheriff of Dumbarton between 1264 and 1266, a post which opened up connections in the westernHighlands. Uilleam was able to marry his younger son Donnchadh toCairistíona Nic Ruaidhrí, daughter of theHebridean chiefAiléan mac Ruaidhrí, a man who had been one of the principal supporters of the Norwegian cause against the Scottish Crown in the 1260s.

When his wife Isabel (also called Elizabeth) died in 1267, Uilleam married Muriel, the daughter ofMaol Íosa II,Mormaer of Strathearn. Uilleam died in 1276, and was succeeded by his sonDomhnall.

• Oram, Richard D., "The Earls and Earldom of Mar, c1150-1300," Steve Boardman and Alasdair Ross (eds.)The Exercise of Power in Medieval Scotland, c.1200-1500, (Dublin/Portland, 2003).