Sir William Blair went to school at Fettes College in Edinburgh and to university at Balliol College, Oxford. Called to the Bar by Lincoln's Inn in 1972, he joined the chambers which is now based at 3 Verulam Buildings. He specialised in the banking and financial aspects of commercial law, and contributed to several books on these areas. He was appointed Queen's Counsel in 1994, and became a Recorder in 1998 and a Deputy High Court Judge in 2003. He was Visiting Professor of Law at the London School of Economics from 1994 to 2018 and Chairman of the Commercial Bar Association from 2003 to 2005. He was appointed a Judge of the Queen's Bench Division in 2008, and nominated as a Judge of the Commercial Court. He was Judge In Charge of the Court from 2016 to 2017. After retiring from the High Court Bench in 2017, he returned to practice as a commercial arbitrator and became a Judge of the Qatar International Court. He also became Professor of Financial Law & Ethics at Queen Mary University, London, with research interests including fintech, financial crime, and financial inclusion.

Sir William continues to write widely on banking and financial services law. His pastimes include travel and walking.