On 13 March 1940, Sir Michael was one of a distinguished company at a joint meeting in the Tudor Hall, Caxton Hall, Westminster, of the East India Association and the Royal Central Asiatic Society. As the meeting was breaking up Udham Singh fired all 6 rounds of a .45 Smith & Wesson revolver into a group of people on the platform of whom O'Dwyer was a part. O'Dwyer was was hit twice in the back, and killed instantly. One bullet passing through his heart and right lung. Another bullet passed through both kidneys.
Lord Zetland, Secretary of State for India, was hit twice although he was only slightly injured, as were Lord Lamington and Sir Louis Dane. The numbers of people killed were not as large as could have been expected, as Udham Singh used 30 year old, poor fitting .44 bullets.
Udham Singh was overpowered before he left the room. His hatred had not been diminished by killing O'Dwyer:
"I did it because I had a grudge against him, he deserved it. I don't belong to any society or anything else. I don't care, I don't mind dying. What is the use of waiting until you get old? That is no good ... Is Zetland dead? He ought to be, I put two in him. I bought the revolver from a soldier in a public house. My parents died when I was 3 or 4 ... Only the one dead, eh? I thought I could get more."
Udham Singh was tried for the murder of Sir Michael O'Dwyer at London's Central Criminal Court during June 1940. He was found guilty and sentenced to death by hanging.
On 31 July 1940, Udham Singh was hanged at Pentonville Prison. As with other executed prisoners, he was buried later that afternoon within the prison grounds.
During the trial, Udham Singh had made a request that his remains be sent back to India, but this was not allowed. In 1975, however, the Government of India, at the instance of the Punjab Government, asked for the return of Udham Singh's remains. Their request was allowed by the UK Government, and his exhumed remains were handed over to representatives of the Indian Government.