The son of Highland parents who had settled in Pollokshaws, John MacLean graduated from Glasgow University and became a teacher in 1900. He joined the Marxist Social Democratic Federation, later the British Socialist Party. In the years before 1914, he concentrated on educational work, organising classes in Marxist economics with the aim of making the working class more socialist. While many flocked to his lectures few joined his party. He did, however, play an active role in stirring up concern about Glasgow's notorious housing conditions. With the outbreak of war in 1914, MacLean denounced the conflict as an imperialist war and parted company with some of the leading figures in the British Socialist Party. He propagated his views through The Vanguard, advocating a combination of strikes and political action as the only way to halt war and to protect workers' rights. In November 1915 he was imprisoned for five days under the Defence of the Realm Act. This cost him his job as a teacher in Pollokshaws. In April 1916 he was found guilty of sedition and sentenced to three years penal servitude.
He was released from prison after fourteen months following many protests over his imprisonment. He renewed his anti-war and revolutionary socialist campaigns, regularly lecturing to miners and others in Lanarkshire and Fife. He was enthusiastic about the Bolshevik revolution in Russia and was appointed the new state's consul-general in Glasgow. He was re-arrested for sedition in April 1918 and remained in prison until December. In the General Election of that month he stood in the Gorbals as a Socialist candidate and received 7,436 votes against 14,247 votes of George Barnes, the sitting member. Despite this, MacLean remained optimistic that a revolutionary situation could be generated in Britain.
In 1920 he broke with the British Socialist Party, but did not join the new British Communist Party, and he became increasingly isolated. He took up the emerging nationalist movement believing in the idea of a Socialist Republic and a distinctive Scottish Workers' Republican Party. Another two spells of imprisonment for sedition followed from May to August 1921 and from October 1921 to October 1922. On his release, he again stood as a candidate for the Gorbals, but his vote halved. Isolation, impoverishment and ill health from the effects of imprisonment were now taking their toll on MacLean and he died in November 1923.