As Attorney-General, he prosecuted William Joyce ("Lord Haw-Haw") and John Amery for treason and also prosecuted Klaus Fuchs and Alan Nunn May, for giving atomic secrets to the Soviet Union and John George Haigh (the acid bath murderer) for murder. He was knighted in 1945 and named Chief Prosecutor for the United Kingdom at Nuremberg. From 1945 to 1949, he was Britain's principal United Nations delegate, though he was recalled in 1948 to lead for the government's interest at the Lynskey tribunal. In 1951, he briefly served as President of the Board of Trade until the Labour government's defeat in the election of that year. He ended his law career the same year, and was expected to become a Tory earning him the nickname "Sir Shortly Floorcross". Instead though he resigned from Parliament in 1958, saying he was tired of party politics. He was made one of Britain's first life peers on February 14, 1959 as Baron Shawcross, of Friston in the County of Sussex, and sat in the House of Lords as a cross-bencher. Because of this change of loyalties away from Labour, his many business interests and, much later, his support for the Social Democratic Party (UK), the 'Floorcross' nickname in the end rang true.
In 1957, he was among a group of eminent British lawyers who founded JUSTICE, the human rights and law reform organisation and he became its first chairman - a position he held until 1972.
He was also instrumental in the foundation of the University of Sussex and served as chancellor of the university from 1965 to 1985.
He had two sons, the author and historian William Shawcross, Hume Shawcross, and a daughter, Dr Joanna Shawcross, by his second wife. He died at home at Cowbeech, East Sussex at the age of 101, the last surviving member of Clement Attlee's government.
He avoided the crusading style of American, Russian and French prosecutors. Shawcross's opening speech, which lasted two days, sought to undermine any belief that the Nuremberg Trials were victor's justice (an exacted vengeance against defeated foes). Instead, he focused on the rule of law and he demonstrated that the laws that the defendants had broken, expressed in international treaties and agreements, were those to which pre-war Germany had been a party. In his closing speech, he ridiculed any notion that any of the defendants could have remained ignorant of the thousands of Germans exterminated because they were old or mentally ill. He used the same argument for the millions of other people "annihilated in the gas chambers or by shooting" and he maintained that each of the 22 defendants was a party to "common murder in its most ruthless forms".
Thus Shawcross' advocacy was instrumental in obtaining convictions against the remaining Nazi leadership, on grounds which were perceived as fair and lawful.
Later, on 10 May 1957, Goddard heard a contempt of court case against Newsweek and W. H. Smith. On April 1 during Adams' trial, they had respectively published and distributed an issue of the magazine containing two paragraphs of material "highly prejudicial to the accused", saying that Adams' victim count could be "as high as 400". Each company was fined £50. Goddard made no mention of his conflict of interest.