Once qualified as a lawyer, Hendy established a law centre, the Newham Rights Centre in East London, and worked there full-time for three years. He then lectured for a year at Middlesex Polytechnic, before returning to the Bar in 1977 and focusing on personal injury and industrial relations cases.
In the mid 1980s he successfully represented Wendy Savage, a consultant obstetrician and gynecologist who was suspended from practice for alleged incompetence. The case lead to him working on others in the area of medical negligence.
In 1991, he was one of four QCs, along with Michael Mansfield, Geoffrey Robertson and Kevin Garnett, acting for the National Union of Mineworkers against claims that had handled funds inappropriately during the miner's strike of 1984–85. In 1992 he represented mining unions at the High Court against attempts to close 31 coal mines.
He retired from head of Old Square Chambers in 2009.
In 2011, The Lawyer labeled him the "barrister champion of the trade union movement", noting that he often assists Unite, ASLEF and the National Union of Rail, Maritime and Transport Workers
His mother was the youngest daughter of the 6th Baron Wynford and his father was "a communist electrician and trade unionist". He describes his father as "a great fighter for human dignity as a trade unionist" and as being the greatest influence on his life. His brother is Peter Hendy CBE, the commissioner of Transport for London.
- "Inquiry: John Hendy QC". The Lawyer. 1997-04-02. Retrieved 2012-01-16.
- Linda Tsang (1999-10-05). "'Seventy-six of my cases have made it into law reports' - Lawyer of the week - Interview - John Hendy QC.". The Times.
- Dowell, Katy (2009-11-11). "The Hendy-man can". The Lawyer. Retrieved 2012-01-16.
- Seumas Milne (17 September 2004). The enemy within: the secret war against the miners. Verso. p. 116. ISBN 978-1-84467-508-1. Retrieved 16 January 2012.
- Mary Fagan, Barrie Clement And Martin Whitfield (1992-10-16). "Judge to rule over mine shutdowns". The Independent. Retrieved 2012-01-16.
- " 2 AC 454 (HL) - Associated Newspapers Ltd v. Wilson; Associated British Ports v. Palmer". Old Square Chambers. Retrieved 2012-04-27.
- "In brief: Old Square Chambers". The Lawyer. 1999-04-10. Retrieved 2012-01-16.
- Chellel, Kit (2009-04-27). "Old Square Chambers head steps down". The Lawyer. Retrieved 2012-01-16.
- "Brothers at arms Features". The Lawyer. 2011-10-10. Retrieved 2012-01-16.
- Peck, Natalie (2012-04-26). "Murdoch endorses ‘conscience clause’ for journalists proposed by the NUJ". Hacked Off. Retrieved 2012-04-27.
- John and Peter Hendy (2007-05-14). "Obituary: Mary Hendy". The Guardian. Retrieved 2012-01-16.