Once dubbed ‘the most dangerous man in Britain’ by the head of British Military Intelligence of the time, MacLean was strongly opposed to Britain’s involvement in the First World War. In a speech he delivered in Edinburgh at the High Court in 1918 he said: ‘I wish no harm to any human being, but I, as one man, am going to exercise my freedom of speech. No human being on the face of the earth, no government is going to take from me my right to speak, my right to protest against wrong, my right to do everything that is for the benefit of mankind’. Under the terms of the Defence of the Realm Act he was later arrested and served over a year in prison.
Overwork, imprisonment, and incidental hunger strikes took their toll on MacLean. Aged 44 he collapsed during a speech and died of pneumonia.
In 1973 a memorial was placed in Pollokshaws which reads: ‘In memory of John MacLean’, ‘Famous pioneer of working-class education. He forged the Scottish link in the Golden Chain of World Socialism’.