As MP Carlile stood unsuccessfully as a
Liberal for
East Flintshire at the general elections of
February and
October 1974. At the
1983 election, standing as a
Liberal Democrat, he became the Member of Parliament for
Montgomeryshire and held the seat until
1997; he also became the leader of his party in Wales. Carlile sat as a Liberal Democrat peer until 2016, when he left the party, stating that he found himself "at odds" with its policies on many matters, including national security issues. It was reported that
civil liberties, especially the
Investigatory Powers Act 2016, were at the core of the disagreement. In 2001 he was appointed the independent reviewer of
terrorism legislation. He continued the role until 2011. From 2001 to 2016 Carlile sat as a deputy judge in the
High Court of Justice. In 2014 Carlile mounted a legal challenge to the UK travel ban on
Maryam Rajavi, leader of the
People's Mujahedin of Iran (MeK) and president-elect of the
National Council of Resistance of Iran. The
Supreme Court decided in favour of the UK government. In 2015, he joined with a cross-party group of peers to reintroduce the
Draft Communications Data Bill, known by its opponents as the "Snoopers' Charter". Carlile was an independent reviewer on the 2015
Assessment on Paramilitary Groups in Northern Ireland. He was vocal in his opposition to the UK coalition government's
Legal Aid, Sentencing and Punishment of Offenders Bill, proposing many amendments. He was one of five Lords who vehemently opposed the introduction of means testing for police advice (to cover the cost of lawyers consulting suspects in police stations). "A single moment of reflection leaves one open-mouthed at the absurdity of this proposal", he said. In 2016 Carlile sat on the founding committee of National Opposition to Windfarms, and sponsored the launch event at the Houses of Parliament. In 2019 Carlile was initially appointed to lead the
independent review of the UK government's
Prevent programme but resigned after a legal challenge was mounted by
Rights Watch UK against his appointment; the petitioner felt strongly that his statements in favour of the strategy would unduly bias his report. The strategy had "become a toxic brand for many within Muslim communities, with some viewing it as a state tool for spying on them." In 2020 Carlile was listed as a bencher on the
Competition Appeal Tribunal, where he sat between 2005 and 2013 as part-time Chair. Carlile is a co-director and co-owner of a strategy and political risks consultancy, SC Strategy Limited with Sir
John Scarlett, the former chief of MI6 and
William Jessett CBE (former Director of Strategy at UK Ministry of Defence. According to the Register of Lords' Interests, Lord Carlile of Berriew was at various times a director of 5 Bell Yard Ltd and the
Wynnstay Group of agricultural feed manufacturers, agricultural goods merchants and fuel oil distributors. He was president of the
Howard League for Penal Reform in 2006–9. ==Penal reform==