He appeared in 100 cases[citation needed] before the Appellate Committee of the House of Lords before its jurisdiction was transferred to the new Supreme Court in October 2009.

He has written on legal matters for The Times, and was co-author with Anthony Lester of Human Rights Law and Practice.

[8] On 29 September 2008, the House of Lords Appointments Commission announced that Pannick had been nominated for a life peerage as a crossbencher.

He acted for the gay servicemen who established in the European Court of Human Rights in 1999 a finding of unlawful dismissal because of their sexual orientation; represented Camelot PLC in the High Court in 2000 and established that the National Lottery Commission had treated it unfairly in rejecting its application to renew its licence to run the National Lottery; acted for the League Against Cruel Sports in defending a challenge to the validity of the Hunting Act 2004; represented a woman who established that she was entitled to be prescribed with the breast cancer drug Herceptin; and was briefed by the Kingdom of Saudi Arabia in its claim to state immunity against allegations of torture.

[16] More recently Pannick acted for AF, a man subject to a control order, establishing that the Home Secretary had a duty to inform him of the essence of the case against him.

He represented The Crown in the Supreme Court in establishing in 2010 that MPs accused of dishonestly claiming expenses were not entitled to the benefit of parliamentary privilege.

[23] In the ruling on the morning of 24 September 2019, the UK supreme court unanimously judged that the Prime Minister Boris Johnson had given unlawful advice to the Queen.

[24] In November 2020, Pannick appeared on behalf of Shamima Begum in the Supreme Court in Begum v Home Secretary, judicial review proceedings brought against the then Home Secretary Sajid Javid's decision not to allow her to return to Britain for legal proceedings regarding the removal of her British citizenship.

[29] Pannick represented the Heung Yee Kuk village organisation in Hong Kong, in a court case to defend the small house policy.