Lansbury helped found, in 1912, the Daily Herald, a socialist newspaper. He became editor just prior to World War I, and used the paper to oppose the war, publishing a headline "War Is Hell" upon outbreak of fighting. In 1922 the Herald, was desperately short of funds and Lansbury reluctantly handed over the paper to the Trades Union Congress and the Labour Party.
As Labour Mayor of Poplar, one of London's poorest boroughs, Lansbury led the Poplar Rates Rebellion in 1921, opposing not only the Government and the London County Council, but leaders of his own party. The borough council, instead of forwarding the precept of collected tax monies to LCC, dispersed the money as aid to the needy. Thirty councillors, including six women, were jailed by the High Court for six weeks. Council meetings during this time were held in Brixton Prison, until the government grew uneasy about the imprisonment and LCC asked the High Court to release the prisoners. A rates revision was achieved and Lansbury returned to Parliament at the 1922 general election, when he regained his old seat of Bromley and Bow.
Lansbury's standing within the Labour party grew continued growing and in 1927 he was elected Chairman of the Parliamentary Labour Party for 1927-28. In 1929 Lansbury became First Commissioner of Works in the second Labour government under Ramsay MacDonald. In this capacity, he was associated with the construction, amongst numerous other public works, of a large open air swimming pool on the Serpentine in Hyde Park, popularly known as 'Lansbury's Lido'. This led to him gaining the popular title "First Commissioner for Good Works".
Two years later the government fell, MacDonald deserted the Labour party to form the National Government and the party went to a massive defeat in the 1931 General Election. The party's new leader Arthur Henderson and nearly every other leading Labour figure were defeated. Lansbury was the one exception and became Chairman of the Parliamentary Labour Party in 1931. The following year Henderson stood down from the leadership of the overall party and Lansbury succeeded him.
The Fulham East by-election in June 1933 was dominated by the issue of re-armament against Nazi Germany, following Germany's withdrawal from the League of Nations. Lansbury, a lifelong Christian pacifist, sent a message to the constituency in his position as Labour Leader:
By 1935 Lansbury found he was denouncing war in the face of calls for military action due to a deteriorating international scene; Italy had invaded Abyssinia and the Labour Party executive were urging the government to call upon the League of Nations to act. He offered his resignation, but was persuaded by colleagues to stay on as leader. A few days after a moving speech in the Commons, Lansbury offered his resignation to Labour MPs and, despite their refusal to accept, he insisted and they reluctantly agreed.
Lansbury was a pacifist and found himself increasingly at odds with the official foreign policy of the party he was leading. On several occasions he offered to resign the leadership but his parliamentary colleagues dissuaded him, not least because there was no clear alternative leader. However in late 1935 the disagreements became more severe and public. Many in the Labour Party, particularly the Trade Union wing led by Ernest Bevin, were pushing for the party to support sanctions against Italy for its aggression against Abyssinia. Lansbury fundamentally disagreed with this. In the weeks leading up to the Labour Party Conference Lansbury's position was weakened when both Lord Ponsonby, the Labour leader in the House of Lords, and the Labour frontbencher and National Executive member Stafford Cripps, widely seen as Lansbury's political heir, resigned from their positions because they too opposed sanctions and felt it would be impossible to lead a party when they were in disagreement with it on the major political issue of the day.
Many wondered how Lansbury's leadership could survive, even though he retained an immense personal popularity. At the Conference this was publicly displayed by delegates, but then during a debate on foreign policy Ernest Bevin launched a withering attack on Lansbury. Heavily defeated in the vote, Lansbury determined to resign as leader. At a meeting of Labour MPs called shortly afterwards there was a great reluctance to accept his resignation, partially out of continued support but also because many Labour MPs feared that the next leader would be Arthur Greenwood, widely seen as heavily aligned to trade unionists like Bevin. In a vote the MPs voted by 38:7 to not accept Lansbury's resignation, but he insisted on stepping down. When it came to selecting a successor (initially envisaged as a temporary position), Greenwood's name was not considered and the party instead unanimously elected Lansbury's deputy, Clement Attlee.
Lansbury was chair of the No More War Movement, chair of the War Resisters' International, 1936-1940, and President of the Peace Pledge Union, 1937-1940. He was a critic of British policy towards the Spanish Civil War and worked with Spanish pacifist José Brocca.
His efforts to prevent World War II led him, under the banner Embassies of Reconciliation, to visit most of the heads of government in Europe, including, controversially, both Adolf Hitler and Benito Mussolini. He also visited U.S. President Franklin D. Roosevelt.
He was an unusually popular politician, an elder statesman with a considerable following. He died of cancer at 81 in Manor House Hospital in North London.
George Lansbury's name and memory live on in the Lansbury Estate and Lansbury Gardens, East London, numerous London street names, and the afore-mentioned Lansbury's Lido that he founded on the Serpentine in London's Hyde Park.