Carswell was Counsel to the Attorney General for Northern Ireland in the years 1969 and 1971, and Senior Crown Counsel in Northern Ireland from 1979 to 1984. In 1984, he became Judge of the High Court of Justice Northern Ireland, a post he held until 1992. He was Lord Justice of Appeal at the Supreme Court of Judicature in Northern Ireland from 1992 to 1997 and further Lord Chief Justice of Northern Ireland from 1997 to 2004. Lord Carswell was made a Queen's Counsel in 1971.