At the age of thirteen he entered King's College, University of Aberdeen, where the first prize in mathematics and physical and moral sciences fell to him.

In his twenty-first year he took orders in the Scottish Episcopal Church, and was ordained to the pastoral charge of a congregation at Pittenweem, Fife, whence he removed in 1790 to Stirling.

In 1808 he was consecrated assistant and successor to the bishop of Brechin, in 1810 was preferred to the sole charge, and in 1816 was elected Primus of the Scottish Episcopal Church, in which capacity he greatly aided in the introduction of many useful reforms, in fostering a more catholic and tolerant spirit, and in cementing a firm alliance with the sister Church of England.

[4] Besides various sermons, Gleig was the author of Directions for the Study of Theology, in a series of letters from a bishop to his son on his admission to holy orders (1827); an edition of Thomas Stackhouse's History of the Bible (1817); and a life of Robertson the historian, prefixed to an edition of his works.

Letters to Alexander Henderson of Edinburgh and John Douglas, bishop of Salisbury, are in the British Museum.