The Mackays claim descent from the Royal House of Moray through the line of Morgund of Pluscarden and were originally known as Clan Morgan. The acceptance of the Mackay's origin deriving from Morgund of Pluscardine is one suggested by Sir Learney as a possible answer to why the clan was called Morgan by Sir Robert Gordon. A possible correct answer to the Morgan question is that the clan name came from an early member named Morgan. Others suggested that Duncan mac Sithig, leader of Clan Morgan, in 1130, was an ancestor of the MacEth/Morgan/Mackay ancestor of our clan. In fact, Duncan was Duncan son of Shaw, possibly of the Duff's of Fife, of the Macintosh line. There may be no connection with these "mac Sithig" Morgans to that of the Clan Mackay.
The clansmen were removed to Sutherland where they rose to a powerful position, at one time owning lands from Drimholisten to Kylescue. Their later title of MacKay comes from a chief so named living at the time of David II. The first record of the name was in 1326 when Gilchrist M'ay, progenitor of the Mackays of Ugadale, made a payment to the Constable of Tarbert. The Mackays supported Bruce and fought with him at Bannockburn and by 1427 the chief, Angus Dubh Mackay was described as leader of "4 000 Strathnaver men". Their fortunes fluctuated over the centuries and many bitter feuds ensued with the Sutherlands and Rosses. In the troubles of the 17th and 18th centuries the Mackays supported the Hanovarian forces against the Jacobites and helped secure the far north for the government. The Mackays of Strathnaver are especially remembered for the famous "Mackay Regiment" raised for the service of the Dutch and Swedish crowns during the 17th century. As a result of this many clansmen settled in Holland and Sweden and gave rise to a number of noble families there. In 1628, Sir Donald Mackay was raised to the peerage of Lord Reay by Charles I. His grandson, Colonel Aeneas Mackay of the Scotch-Dutch Brigade, married the heiress of the Baron van Haefton. The Mackays suffered badly in the Strathnaver clearances between 1815 and 1818 and finally in 1829 the Reay estate was sold to the Sutherland family and in 1875 the chiefship passed to Baron Mackay van Opermet who became 10th Lord Reay. His nephew Baron Aeneas Mackay, prime minister of the Netherlands was the great grandfather of the present chief.
The Clan Mackay Chiefship is vested in the Lords Reay who also hold the title Barons Van Ophemert in Holland. Donald MacKay, Chief of MacKay, was knighted first Lord Reay.
Donald MacKay, First Lord Reay, was knighted Baronet of Nova Scotia when he acquired Anticosti Island in (then) Nova Scotia. Baronet of Nova Scotia is a hereditary title; Hugh William Mackay, 14th Lord Reay, present Chief of MacKay, is 14th Baronet of Nova Scotia.