常任上訴法官
Mackay was appointed a judge of the Court of Session and High Court of Justiciary, Scotland's highest courts, in March 2000. Mackay was also one of five members of the House of Lords, in addition to the twelve Lords of Appeal in Ordinary, eligible to form the quorum of the House required to hear and determine judicial business under ss.5&amp;25 of the Appellate Jurisdiction Act 1876. In October 2009 the judicial functions of the House of Lords were transferred to the new Supreme Court of the United Kingdom under Part 3 of the Constitutional Reform Act 2005, with the twelve Lords of Appeal in Ordinary becoming the inaugural Justices of the Court. While ss.38 and 39 allow for additional judges to sit in the Court, Mackay's position as a serving judge of the Outer House of the Court of Session excluded him from both of these provisions.