William Gallacher was born on 25 December 1881 in Paisley, Scotland. He became the family breadwinner following his father’s death in 1888, taking part-time jobs to support his mother and siblings. In 1905 he became a member of the Independent Labour Party, but later joined the more radical Social Democratic Federation (which became the British Socialist Party), where he befriended John Maclean and John R. Campbell. An engineering fitter by trade, Gallacher became a shop steward at the Albion Motor Works in Scotstoun in 1914, later becoming chairman of the Clyde Workers Committee (CWC), formed to campaign against the Munitions Act which forbade engineers from leaving the works where they were employed.

Other Glasgow socialists with whom he worked at this time included David Kirkwood, John Wheatley, James Maxton, Emanuel Shinwell, John Muir, Tom Johnston, Jimmie Stewart, Neil Maclean, George Hardie, George Buchanan and James Welsh.

In February 1916 a CWC protest at Beardmores Munitions Works in Parkhead took place, which saw David Kirkwood, Arthur McManus, and other members of the CWC arrested and deported from Glasgow to Edinburgh the following month. That same year, CWC’s journal, The Worker, was prosecuted under the Defence of the Realm Act for an article criticising the Great War. Gallacher, who was strongly opposed, and editor John Muir were both arrested, found guilty and sentenced; the former for 6 months and Muir for 1 year.

Following the war, Gallacher became heavily involved in the struggle for a 40-hour week, which saw him arrested once again in 1919, following a trade union protest in George Square in Glasgow on 31 January. He served 5 months.

Gallacher subsequently travelled to Moscow in 1920 to represent the CWC at the Second Congress of the Communist International, where he met Vladimir Lenin, who convinced attendees of the need for a Communist Party in Britain. Upon his return he was one of the early members of the newly-formed Communist Party of Great Britain (CPGB). He was involved in forming the Communist Labour Party, a transitional body based in Scotland, which served to bring most of the membership of the Socialist Labour Party into the CPGB in early 1921. He also served as a member of its central committee from 1922 to 1963.

Gallacher ran for election to the House of Commons in 1922 and 1923, for Dundee, but was unsuccessful. In August 1925, alongside Tom Bell, Jack Murphy, Wal Hannington, Ernie Cant, Tom Wintringham, Harry Pollitt, Albert Inkpin, Arthur McManus, William Rust, Robin Page Arnot and John Campbell, he was was arrested for being a member of the Communist Party of Great Britain and charged with violation of the Mutiny Act of 1797. Gallacher, Hannington, Inkpin, Pollitt and Rust, were sentenced to twelve months’ imprisonment and the others to six months.

Gallacher was elected Communist MP for West Fife in 1935, and for East Fife in 1945, and served his constituencies for 15 years until 1950.

In 1936 he joined members of the Labour Party in seeking military support for the Spanish Popular Front government fighting against General Francisco Franco and the Nationalist Army.

In the House of Commons Gallacher associated with left-wing members including John Platts-Mills, Konni Zilliacus, Lester Hutchinson, Ian Mikardo, Barbara Castle, Sydney Silverman, Geoffrey Bing, Emrys Hughes, D. N. Pritt, Leslie Solley, and William Warbey.

Gallacher's opposition to the Second World War, the Cold War, and the North Atlantic Treaty Organization (NATO) made him an unpopular figure in post-war Britain, contributing to his electoral defeat in the 1950 General Election. He remained active in politics and served as chairman of the Communist Party from 1950 until 1956, when he became President, serving until 1963/65.

William Gallacher died on 12th August 1965. He had married his wife Jean (née Roy) in 1913. The couple lost their two children in infancy. Following his own brother's death, William and Jean adopted their two nephews, John and Paul, both of whom died during the Second World War.