He qualified as a solicitor in 1978, became an advocate in 1983 and took silk in 1995. He practised at Caesar & Howie from 1978 to 1982 and as an advocate from 1983 to 1997, building up a practice in civil and particularly planning law.

He acted as an advocate depute from 1993 to 1995 and was appointed solicitor general for Scotland (for the UK government) in 1997 and for the Scottish Executive in 1999.
Between 2000 and 2006 he was lord advocate.

His time in office saw the devolution of legislative responsibility to the new Scottish Parliament and the introduction of the European Convention on Human Rights into domestic law. He brought in significant reforms to the Crown Office was responsible for the prosecution of the Lockerbie trial.

Lord Boyd returned to practice as a solicitor advocate in 2007 joining Dundas & Wilson Solicitors as a consultant and head of public law. He was appointed a privy councillor in 2000 and became a life peer in 2006. In June 2012 he was appointed as a judge of the Supreme Courts and in 2019 he was appointed vice-president of the Investigatory Powers Tribunal in London.