Horniman served as vice president of the Home Rule League under Annie Besant and called for a satyagraha campaign against the Rowlatt Act in 1919 through The Bombay Chronicle and at public meetings. When Gandhi formed the Satyagraha Sabha to launch a national campaign against the Rowlatt Act, Horniman was made its vice-president. His decision to print an unofficial, smuggled report on the Jallianwala Bagh massacre in defiance of government censorship resulted in his deportation to the United Kingdom by the British colonial government.