On the bench, he presided over the notorious ABC trial in 1978 in which John Berry, a former corporal in the Royal Corps of Signals, was found guilty under the unpopular section two of the Official Secrets Act of passing information to the journalists Duncan Campbell and John Aubrey. Campbell had then written in Time Out that British and American forces regularly monitored the airwaves, and named the sites from which monitoring occurred. Many were surprised when the reporters were given conditional discharges and Barry a short suspended sentence by Mars-Jones.
In 1983 he took the unusual step of banning both Jews and Muslims from the jury in the case of two Jordanian students and an Iraqi businessman for the attempted assassination of the Israeli Ambassador, who had been shot through the head.
Three years later he sentenced Nezar Hindawi to 45 years' imprisonment, the longest single term imposed in Britain, after he had been found guilty of trying to blow up an Israeli aeroplane by planting a bomb in his pregnant girlfriend's luggage. The trial had been a stormy one and Gilbert Gray QC, acting for Hindawi, told the jury that "another nation may take retribution" if they convicted. Mars-Jones expressed surprise that Gray had suggested that armed conflict could break out following the verdict. "It is a suggestion which should never have been made," he added, telling the jury their decision would not alter the attitude of foreign powers or Britain. "Keep your cool," he said.
In the case of the actor John Bindon, acquitted of what the prosecution had alleged was a contract killing in Putney, it was thought that Mars- Jones had been sympathetic towards Bindon in his summing-up and unhappy with the ragbag of witnesses produced by the prosecution.
On the civil side, in 1984 he ordered Harrow Borough Council to pay damages to a schoolgirl who had put on excessive weight after she had broken a leg jumping a hurdle in a physical education class. The girl had put on some 70lb and Mars-Jones said, "She is entitled to compensation for the fact that she became so grossly overweight, which certainly made her look less attractive. Her vast increase in size was a direct result of her inactivity. She was bored and unhappy, so she took to eating sweets and more hospital food than her body required."
He also made a highly controversial decision when, in 1982, he awarded David and Lucille White, a middle-aged Jamaican couple, substantial damages for what he described "monstrous, wicked and shameful" police conduct. He accused the police of a five-year cover-up of what he described as their "brutal, savage and sustained variety of assaults", which had occurred when the police had raided the couple's home in Stoke Newington, London.
Born in 1915 in Llansannan, where his father, later chairman of Denbighshire County Council, ran the village post office, Mars-Jones attended Denbigh School. He obtained a First in Law at University College, Aberystwyth, where he was not only president of the Students' Council and the Central Students' Council but was also regarded as a great entertainer. Throughout his life he was a brilliant raconteur and mimic. He was also an accomplished musician, playing the guitar, piano and, more unusually, the ukulele.
He then attended St John's College, Cambridge, where he was a member of the Cambridge Footlights. He read for the Bar, joining Gray's Inn and, when the Second World War broke out, the Navy, becoming a lieutenant-commander. He became a Bencher of the Inn in 1964 and was its Treasurer in 1982.
Immediately after the war he stood as Labour candidate for West Denbigh, losing to the sitting Conservative member. He then abandoned any political ambitions and threw himself into life on the Wales and Chester Circuit, first as its junior and then in later life as its leader and finally presiding judge. He was elected an honorary life member shortly before he retired in 1990.
A fluent Welsh speaker, he was a man who had a reputation for being a fighter and thereby enjoying the following of a large number of solicitors, he took silk in 1957. He prosecuted the notorious Moors Murders trial and was appointed to the High Court bench in 1969. He had already served as Recorder of Birkenhead, of Swansea and of Cardiff from 1968. He was also Deputy Chairman of Denbighshire Quarter Sessions from 1962 to 1968.
In 1964 he headed the Home Office Inquiry into allegations against Metropolitan Police Officers and he also chaired the Home Secretary's Advisory Council on the Penal System in 1966.
He married Sheila Cobon in 1947 and they had three sons, including the writer Adam Mars-Jones, who acted as his father's Marshal at the Black Panther trial for the killing of Leslie Whittle. In a fictionalised account he described his father as "wizened", something about which Bill Mars- Jones took friendly umbrage. After his wife's death last year, Mars-Jones became increasingly frail, rarely visiting the Garrick Club, of which he was a long-standing member.
William Lloyd Mars-Jones, judge: born Llansannan, Denbighshire 4 September 1915; called to the Bar, Gray's Inn 1939, Bencher 1964, Treasurer 1982; MBE 1945; Recorder of Birkenhead 1959-65, Swansea 1965-68, Cardiff 1968- 69; Judge of the High Court of Justice, Queen's Bench Division 1969-90; Kt 1969; married 1947 Sheila Cobon (died 1998; three sons); died London 10 January 1999.