Margaret, Countess of Mar was born in Scotland to Domhnall II, Earl of Mar (c1302-1332) and Isabella Stewart (-c1332) and died circa 1391 Scotland of unspecified causes. She married William Douglas, 1st Earl of Douglas (c1327-1384) .
Margaret of Mar (died c. 1391) was Countess of Mar, an ancient Earldom in Scotland, in her own right.
She was a daughter of Domhnall II of Mar and after the death of her childless brother Thomas became Countess of Mar. She had married William Douglas, 1st Earl of Douglas, who was succeeded by their son, James Douglas, 2nd Earl of Douglas and Earl of Mar and Garioch in right of his mother, and when he fell, leading the Scots at the battle of Otterburn.
She was succeeded by her daughter, Isabel, who became Countess of Mar, possessed the Lordship of the Garioch, and also became the Countesss of the unentailed lands of the House of Douglas. - http://en.wikipedia.org/wiki/Margaret,_Countess_of_Mar
Children
| Name | Birth | Death | Joined with |
| James Douglas, 2nd Earl of Douglas (c1358-1388) | 1358 Scotland, United Kingdom | 14 August 1388 Otterburn, Northumberland, England, United Kingdom | Isabella Stewart (c1348-c1410) |
| Isabel Douglas, Countess of Mar (c1360-1408) | |||
Siblings
| Name | Birth | Death | Joined with |
| Thomas, Earl of Mar (c1330-1377) | |||
| Margaret, Countess of Mar (-c1391) | Scotland | 1391 Scotland | William Douglas, 1st Earl of Douglas (c1327-1384) |