McCluskey was briefly Standing Junior Counsel, a legal advisor to a government department, to the
Ministry of Power in Scotland in 1962. As an advocate, he defended
Paul McCartney against drugs charges in March 1973 when a trial took place in Campbelltown. He caused uproar in court when he asked if McCartney could have "time to pay" a £30 fine. McCluskey was appointed
Solicitor General for Scotland in March 1974 in the new
Labour government of
Harold Wilson. On 29 September 1976 he was created a life peer as
Baron McCluskey, of Churchhill in the District of the City of Edinburgh. This was so that he could help steer the
Devolution bill through the House of Lords. In December 1984, McCluskey was appointed a
Senator of the College of Justice, a judge of the
Court of Session and
High Court of Justiciary, Scotland's
supreme courts. In 1989 the
University of Dundee gave him an honorary doctorate. In 1992 he presided over the trial of
Paul Ferris, who was accused of a gangland murder. The trial lasted 54 days and cost £4 million, which at the time was the longest and most expensive criminal trial in Scottish legal history. He was involved with helping safeguard the independence of the judiciary from a provision contained in the bill that became the
Scotland Act 1998. In March 1999 he presided over the first trial held after the
murder of Surjit Singh Chhokar at the
Glasgow High Court, although that had only led to a conviction of assault. McCluskey was Scotland's longest-serving judge at the time, and was highly critical that only one person had appeared in the dock. In July of that year, he addressed the
Law Society of Scotland's 50th annual conference and suggested that a Royal Commission should look at sentencing of drug offenders. From 1988 to 2005, he was editor of Butterworth's
Scottish Criminal Law and Practice series. He was a friend of the Labour leader
John Smith, and played tennis with him. He was a trustee of the John Smith Memorial Trust, and for a time chaired the trustee board. He also chaired the Scottish Association of Mental Health ==Retirement==