As lord chief justice (1913–21), Isaacs presided over the trial for treason of the Irish patriot Sir Roger Casement (1916). During World War I he worked to strengthen Anglo-American relations, and he headed an Anglo-French delegation that negotiated a war loan of $500 million from the United States. Reading (who had been created baron in 1914, viscount in 1916, and earl in 1917) retained his lord chief justiceship while serving as ambassador to the United States (1918–19).

Always be shorter than anybody dared to hope.

Rufus Isaacs was the first commoner to rise to the rank of marquess since the Duke of Wellington. Born into a lively Jewish family engaged in the London fruit trade, he went on to become a brilliant QC, Lord Chief Justice, Special Wartime Ambassador to the United States, Viceroy oflndia and Foreign Secretary.
A stalwart of the Liberal Party, he first won a Commons seat in 1904 and was soon a Cabinet Minister, trusted by Asquith, and at the same time very close to the controversial Lloyd George. His life abounded in paradox: leaving school at fourteen, he was later to hold his own with the best minds in international politics; a rebellious child, he came to personify the grave majesty of the law; once a ship's boy, he ruled over the Indian Empire; 'hammered' as a stock jobber, he became a respected Liberal elder statesman; smeared during the Marconi scandal and subject to anti-semitic attack, he was promptly appointed Lord Chief Justice and raised to the peerage as Lord Reading.

Isaacs was one of several high-ranking members of the Liberal government accused of involvement in the Marconi scandal. An article published in Le Matin on 14 February 1913 alleged corruption in the award of a government contract to the Marconi Company and insider trading in Marconi's shares, implicating a number of sitting government ministers, including Lloyd George, the Chancellor of the Exchequer; Isaacs, then Attorney General; Herbert Samuel, Postmaster General; and the Treasurer of the Liberal Party, the Master of Elibank, Lord Murray.
The allegations included the fact that Isaacs' brother, Godfrey Isaacs, was managing director of the Marconi company at the time that the cabinet, in which Isaacs sat, awarded Marconi the contract. Isaacs and Samuels sued Le Matin for libel, and as a result, the journal apologised and printed a complete retraction in its 18 February 1913 issue.
The factual matters were at least partly resolved by a parliamentary select committee investigation, which issued three reports: all found that Isaacs and others had purchased shares in the American Marconi company, but while the fellow-Liberal members of the committee cleared the ministers of all blame, the opposition members reported that Isaacs and others had acted with "grave impropriety".[11] It was not made public during the trial that these shares had been made available through Isaacs's brother at a favourable price

Along with his in-law Alfred Mond (father of his daughter in-law) and Herbert Samuel, Isaacs was a founding chairman of the Palestine Electric Corporation, precursor to the Israel Electric Corporation in the British Mandate of Palestine. The Reading Power Station in Tel Aviv, Israel was named in his honour.