Domhnall I of Mar - Domhnall mac Uilleim (Donald, William's son) - was the sixth known Mormaer (Earl) of Mar, ruling from 1276 until his death somewhere between 1297 and 1302.

Sir Donald, 6th Earl of Mar was invested as a Knight on 29 September 1270. He succeeded to the title of 6th Earl of Mar on 25 July 1281.

Domhnall was a strong supporter of the Bruce cause during the crisis of the late 13th century. He was at Norham in 1292, probably in the camp of Robert de Brus, then Mormaer or Earl of Carrick.
He was married to Helen (sometimes called Ellen), the daughter of Llywelyn ap Iorwerth, who herself had previously been married to Mormaer Maol Chaluim II of Fife. By Helen, he had three sons, including his successor Gartnait, and two daughters. His daughter Isabella of Mar was the first wife of Robert I of Scotland and mother of Marjorie Bruce.

The last record of a living Domhnall comes from 1297, and the earliest record of his son Gartnait as Mormaer is from 1305, creating the range of Domhnall's possible year of death to somewhere in between these two points. However, a document dating to 1302,1 containing terms of reconciliation between Edward I and Robert, stipulates that Robert should act as warden of Gartnait, implying that Domhnall had just died.