Beatson was called to the Bar at the Inner Temple in 1972, becoming a member of the governing council in 1993 as an honorary Bencher. He was Law Commissioner for England and Wales for five years from July 1989 to 1994, working on contact and commercial law, civil evidence, damages, administrative law, and financial services. He rejoined Essex Court Chambers in 1994, and appointed a Queen's Counsel in 1998.
He was a law lecturer at the University of Bristol 1972–73, then became a law tutor at Merton College, Oxford until 1994. He was the founding director of the Centre for Public Law (1997–2001) and is an Honorary Fellow of St John's College, Cambridge. He was Rouse Ball Professor of English Law at the University of Cambridge from 1993 to 2003. Lord Falconer called him an "outstanding academic lawyer".
In July 2001 he became a fellow of the British Academy and he was President of the British Academy of Forensic Sciences 2007–09. In 2012 Beatson was a member of the advisory group that produced A Restatement of the English Law of Unjust Enrichment.
Beatson was a Crown Court Recorder in 1994 and a Deputy High Court Judge in 1999. He was appointed to the High Court on 29 April 2003, receiving the customary knighthood, and was assigned to the Queen's Bench Division. On 26 July 2012, it was announced he would be appointed a Lord Justice of Appeal to fill a forthcoming vacancy.