Gallacher was opposed to the United Kingdom becoming involved in World War I and was president of the Clyde Workers' Committee, an organisation that had been formed to campaign against the Munitions Act, which forbade engineers from leaving the works where they were employed and to organise the Clydeside workers in general. David Lloyd George and Arthur Henderson met Gallacher and the Clyde Workers' Committee in Glasgow but they were unwilling to back down on the issue.
In 1916 the Clyde Workers' Committee journal, The Worker, was prosecuted under the Defence of the Realm Act for an article criticizing the war. Gallacher and John Muir, the editor were both found guilty and sent to prison. Gallacher for six months and Muir for a year.
After the war Gallacher was involved in the struggle for improving workers conditions. The police broke up mass rally of striking workers at George Square, Glasgow on January 31, 1919. The then Liberal government greatly over-reacted to the strike, thinking that a Bolshevik insurrection was about to begin on Clydeside, and sent English troops and tanks onto the streets of Glasgow to control the situation. Whilst revolution was the furthest thing from the minds of the trade union leaders of the day, Gallacher later claimed that they should have marched to the barracks in the Maryhill district of the city and encouraged the Scottish troops there to leave them and join the workers against the government.
The union leaders of the were arrested and charged with "instigating and inciting large crowds of persons to form part of a riotous mob". Gallacher was returned to jail, being sentenced to five months.
Gallacher joined the Communist Party of Great Britain (CPGB) and attempted to be elected to the House of Commons at Dundee (1922 and 1923), West Fife (1929 and 1931) and Shipley (1930). He was eventually elected to represent West Fife in 1935.
In 1936 Gallacher joined members of the Labour Party such as Stafford Cripps and Aneurin Bevan in arguing for giving military help to the Spanish Popular Front government fighting against Franco's Fascist forces in the Spanish Civil War.
Gallacher lost his West Fife seat at the 1950 General Election but remained in politics and served as President of the CPGB between 1956 and 1963.
William Gallacher died on August 12, 1965. He remains to this day only one of two Communist Members of Parliament ever elected in the UK. The author of several books, his autobiography, The Chosen Few, was published in 1940. He also wrote a book about his experiences during the first world war, Revolt on the Clyde.