Mackay was elected to the Faculty of Advocates in 1955. He was appointed a Queen's Counsel in 1965. He was Sheriff Principal for Renfrew and Argyll from 1972 to 1974. In 1973 he became Vice-Dean of the Faculty on Advocates and from 1976 until 1979 served as its Dean, the leader of the Scots bar. 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. Since his retirement, Mackay has sat in the House of Lords and was Commissary to the University of Cambridge until 2016. He is the Editor-in-Chief of Halsbury's Laws of England, the major legal work which states the law of England, first published in 1907; the post is usually held by a former Lord Chancellor. He is also a Senior Fellow of , a Christian nonprofit that supports the renewal of society through the development of leaders.
Religion
Mackay was raised a member of the Free Presbyterian Church of Scotland; as an adult he was an elder of the church. The church forbids its members to attend Roman Catholic religious services; nevertheless Mackay attended two Catholic funeral masses for members of the judiciary. Following the second mass Mackay was called before a church synod where he denied that he had broken the church's prohibition of showing "support for the doctrine of Roman Catholicism", saying "I went there purely with the purpose of paying my respects to my dead colleagues." The church suspended Mackay from the eldership and from membership. 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", and later withdrew from the church. The dispute precipitated a schism, leading to the formation of the Associated Presbyterian Church. Mackay did not, however, initially join the new communion, but now worships with their Inverness congregation. in Bangalore by Lord Mackay As a Presbyterian, Mackay was a firm believer in moderation. 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. One of the lawyers in attendance contemplated the pot and remarked, "I see your Lordship keeps a bee." Mackay is also the Honorary President of the Scottish Bible Society. He supported the society's programme to send a Bible to every court in Scotland and wrote in support of "The Bible in Scots Law", a pamphlet it distributed to Scottish lawyers which described the Bible as a "foundational source book for Scotland's legal system". 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.
