The Erskine's derived their surname from their barony in Renfrewshire. The family may have come from Flanders, descending from Gilbert de Ghent.
A Robert Erskine married the niece of Robert the Bruce, her brother Donald being the 8th Earl of Mar; thus giving the Erskine's the claim to the title when Donald's line of descent died out. However, Isabella, Donald's granddaughter, gave the title of Earl to her second husband, Alexander Stewart, but he resigned the Earldom to the crown in 1408 when Isabella died. The crown later gave back his title in 1426.
The grandson of that Robert Erskine - another Robert, the same Sir Robert that obtained Garscadden in 1369 - was recognised as the heir to the earldom in 1395. (He was also appointed the Constable of Stirling Castle by David II and was made the Lord Great Chamberlain of Scotland in 1350. After obtaining Garscadden he was also to become the keeper of Dumbarton Castle, but this distinction fell to the Dennistoun family on the succession of Robert II.) When Alexander Stewart died in 1435, the crown took back the earldom and various members of the Royal family assumed the title between 1459 and 1562.
Sir Robert became a Lord of Parliament, the 1st Lord Erskine and Ambassador. His descendents were later to assume the title of the Earl of Mar as they had always claimed when, in 1626, the Court of Session validated the crown's grant to Alexander Stewart in 1426 and thus the Erskine's claim.
The grandson of the 1st Lord Erskine, another Sir Robert, passed the estate of Garscadden to Patrick Galbraith, his armour-bearer, on the 8th June 1444.