This surname is derived from the Gaelic which meant ‘son of the grey lad’. Some associated spellings are Galashan, Glassin, MacGlass, MacGlossen and MacGolash.

The MacGlashan sept of Clan Donnachaidh are said to be "descended from the old Stewart family of Ballechin". The Adam’s reference goes on to say that due to "some family disagreement, these Stewarts renounced their family name, and adopted in place of it that of MacGlashan, by which name they have ever since been known." When the old Stewart line of the Earls of Atholl died out in 1625, the earldom reverted to the Crown. The Crown bestowed the earldom in 1629 to the current Murray ducal line. At that time "the MacGlashans appear to have become followers of the Clan Donnachie". The reference states that "Some of them who settled in the south Anglicised their names as Gray."

In 1600, Sir James Stewart of Ballechin gave a charter of the lands and mill of Pitlochry to his cousin, John Butter of Killivoulin.

The estate of Ballechin is located in the Logierait parish of Perthshire on the north side of the River Tay, approximately 4 miles southeast of Pitlochry. The village of Logierait is situated at the confluence of the rivers Tay and Tummel. Logierait was the seat of the regality court of the Earls of Atholl. Following the River Tay west toward Loch Tay were the Robertsons of Fearnan, Lawers and other estates.

An extract from the Session Minutes records states that on April 11, 1744, Susan McGlashan, wife of Donald Robertson in Easter Kindrochit breached the Sabbath by leading a horse with a load of baggage on its back from the boat of Apersuanie to their house. She was summoned to answer for said breach.

The inn at Blair Atholl in 1746 was called the MacGlashan Inn after the proprietor.

The name MacGlashan is also a sept of Clan Mackintosh, part of the Clan Chattan confederation. That clan area is located in the north, southeast of Inverness near Culloden.

References:
Adam, Frank, "The Clans, Septs and Regiments of the Scottish Highlands", W. & A.K. Johnston Limited, Edinburgh, 1908, p. 180.
Kerr, John, "Church and Social History of Atholl", Perth, 1998, p. 32.
McNie, Alan, "Clan Robertson", Cascade Publishing Co., Jedburgh, 1989, p. 29.
Mitchell, Hugh, "Pitlochry District", Pitlochry, 1923, p. 142.
Robertson, James A., "Gaelic Topography of Scotland", Edinburgh, 1869, pp. 447.
Smith, Philip D., "Tartan for Me".