In 1928, British colonial authorities in India detained and held J.W. Johnstone, a US citizen, for nearly a month before deporting him first to Europe and then back to the USA. Johnstone’s eventual arrest and deportation became a major ‘affair’ with far-reaching implications for India, the British Empire and even the USA. In the weeks after Johnstone’s arrival, the colonial state launched an extensive and worldwide investigation into his identity and potential ties to communism. In analyzing the story of the Johnstone affair, this article highlights British colonial anxieties and preoccupations with the spread of international communism in interwar India. Moreover, this article also argues that the response to the Johnstone arrest – in India and the United States of America especially – produced a number of unintended consequences. Both the American working class movement and the Indian trade unionist movement appropriated the Johnstone affair to call for global solidarities against the oppression of British and US imperialism worldwide.