He won a scholarship to George Heriot's School,[3] and then studied mathematics and physics at the University of Edinburgh, receiving a joint MA in 1948.

[1] In 1979, Mackay was appointed Lord Advocate, the senior law officer in Scotland, and was created a life peer as Baron Mackay of Clashfern, of Eddrachillis in the District of Sutherland, taking his territorial designation from his father's birthplace, a cottage beside Loch na Claise Fearna.

[5] He is also a senior fellow of The Trinity Forum, a Christian nonprofit organisation that supports the renewal of society through the development of leaders.

[1] The church forbids its members to attend Catholic religious services; nevertheless Mackay attended two Catholic funeral masses for members of the judiciary (for Charles Ritchie Russell in 1986,[4] and again for John Wheatley in 1988).

[1] The synod met again in Glasgow in 1989 to review the decision; the meeting asked Mackay to undertake not to attend further Catholic services, but he announced "I have no intention of giving any such undertaking as that for which the synod has asked",[8] and later withdrew from the church.

At a gathering for the Faculty of Advocates, Mackay had laid on a spread of tea and toast, complete with a tiny pot of honey.

[12] He is a strict sabbatarian, refusing to work or travel on a Sunday, or even to give an interview if there is a chance it could be rebroadcast on the sabbath.

[13] In 2007 the Queen appointed him to the office of Lord Clerk Register, replacing David Charteris, 12th Earl of Wemyss.