Cranston was a professor at the LSE from 1992-7 and the holder of the Cassell Chair in Commercial Law from 1993 to 1997. Before that he held academic posts in the UK and Australia and the Sir John Lubbock chair in Banking Law at QMW, being a professor of Law at Queen Mary and Westfield College from 1986-91.
He was appointed as a High Court judge in October 2007, assigned to the Queen's Bench Division .
On 14 December 2007, sitting in the Court of Appeal, Criminal Division, Mr Justice Cranston (together with Mr Justice Cooke) heard an appeal against sentence by a woman who had pleaded guilty to a charge of dangerous driving, and other matters. Having stolen goods from a Tesco store she had driven her car straight at a security guard, who jumped onto the bonnet to avoid being hit. The appellant had screamed at him to get off, saying she would otherwise kill him. He eventually jumped off. A police car, its lights blazing and sirens switched on, pursued the appellant out of Leamington as she failed to stop. She was ultimately stopped by another police car blocking a slip-road. The sentencing judge had concluded that her actions were "deliberate, sustained and highly dangerous".
Mr Justice Cranston described this conduct as "a bad case of dangerous driving but not the worst", and reduced the judge's sentence of 15 months imprisonment to one of 9 months. .