On hearing the news of the massacre at Jalianwala Bagh, Udham Singh did not attend his school but took a train to Amritsar and visited the spot. There he stood for several minutes in a trance, picked up the soil, rubbed it on his forehead and put some in a phial. He fasted that night. According to his family members, he placed fresh flowers on the holy dust every morning drawing inspiration from it. One early morning he went to the river Ravi and pledged that he would colour its waters with the colour of his blood, to create what he called the freedom's flood. With this aim in view in 1921, he first landed in Africa. From there he went to America to meet the Indian revolutionaries working for the liberation of the motherland.

In 1923, he landed in England. In 1928 he returned to India in response to an urgent call from Bhagat Singh. When he reached Lahore, he was detained for violation of the Arms Act. He was released after four years rigorous imprisonment. In 1935, he escaped to Germany by giving a slip to the police. From Berlin, he reached Paris and purchased a revolver. He kept it ready for action and managed to reach London again. The opportunity came on March 13, 1940 when Sir Michael O'Dwyer was to speak at a seminar in Caxton Hall. As Sir Michael turned to resume his seat after his speech, Udham Singh pulled out his revolver and fired at him. Sir Michael died without a shriek. Udham Singh was arrested and produced before the court on April 2, 1940. The court passed death sentence on him and he was hanged at Pentonville Prison, London on July 31, 1940.