In January 1919 the Anglo-Irish war started and Tomás Mac Curtain became an officer in the IRA. On 20 March 1920, his 36th birthday, Mac Curtain was shot dead in front of his wife and son by a group of men with blackened faces, who were found to be members of the Royal Irish Constabulary (RIC) by the official inquest into the event.In the wake of the killing which was in revenge for the shooting of a policeman, Mac Curtain's house in the city's Blackpool area, was ransacked.
The killing caused widespread public outrage. The coroner's inquest passed a verdict of wilful murder against British Prime Minister Lloyd George and against certain members of the RIC. Michael Collins later ordered his personal assassination squad to hunt down and kill the police officers involved in the attack. RIC District Inspector Oswald Swanzy, who had ordered the attack, was fatally shot with Mac Curtain's revolver while leaving a Protestant church in Lisburn, County Antrim on 22 August 1920, sparking a "pogrom" against the Catholic residents of the town. Mac Curtain is buried in St. Finbarr's Cemetery, Cork.
His successor to the position of Lord Mayor, Terence MacSwiney, died while on hunger strike in Brixton prison, London.