Mallin enrolled as a soldier with the British Army’s 21st Royal Scots Fusiliers on 21 October 1889. During the early years of his service he was stationed in Great Britain and Ireland. His Regiment was sent to India in 1896, where he served out the remainder of his almost fourteen-year career, taking part in the Tirah Campaign. It was during his time in India that he became radicalised. In 1897, when asked to donate to the memorial fund for Queen Victoria’s jubilee year he refused because ‘he could not subscribe as the English monarch had taken an oath to uphold the Protestant faith’. Mallin’s brother, Thomas, later suggested that incidents such as this kept him from being promoted any higher than a drummer. He was awarded the India Medal of 1895 with the Punjab Frontier and Tirah clasps 1897–98.

Mallin married Agnes Hickey, whom he had met during his early home service in the Army, in 1903. They had three sons and two daughters, the youngest born after Mallin’s execution.

His youngest son, Fr. Joseph Mallin SJ, born on 13 September 1913, was a Jesuit priest and teacher in Hong Kong. He was the last surviving child of those executed in the Rising until his death in Hong Kong on 1 April 2018 at the age of 104.

He became active in politics, and was the Secretary of the Socialist Party of Ireland. He progressed to become a leading official in the silk weavers’ union. During the 1913 Lockout, he led a strike of silk workers at the Hanbury Lane factory. The strike lasted for thirteen weeks, with Mallin an effective negotiator on behalf of the strikers. He was appointed second-in-command and chief training officer of the Irish Citizen Army (ICA), which was formed to protect workers from the Dublin Metropolitan Police (DMP) and from employer-funded gangs of strike-breakers. Under the tutelage of Mallin and James Connolly, the ICA became an effective military force. He was also appointed chief of staff of the ICA in October 1914.

When Connolly was inducted into the Irish Republican Brotherhood (IRB) in January 1916, Mallin began preparing ICA members for the imminent armed revolution. In the week before the operation he communicated orders to the ICA members throughout the city. On Easter Monday, Mallin departed from Liberty Hall at 11:30 am to take up a position at St Stephen’s Green with a small force of ICA men and women. Upon arriving at the park they ordered civilians out of it, dug trenches, erected kitchen and first aid stations, and built barricades in the surrounding streets. Constance Markievicz arrived and was originally thought to have been appointed Mallin’s second-in-command, but later evidence pointed to this role belonging to Captain Christopher Poole, with Markievicz being third-in-command.

Mallin obeyed the order and surrendered his position to Captain H.E. De Courcy-Wheeler, Staff Captain to General Lowe, acting Commander of British troops in Ireland. Mallin and the men and women under his command were arrested and taken prisoner. They were escorted first to Ship Street Barracks at Dublin Castle then on to Richmond Barracks, at Inchicore where Mallin was separated for court-martial. Mallin was court-martialled on 5 May, found guilty of the charge of treason and he was executed by firing squad in the stonebreaker’s yard at Kilmainham Gaol at sunrise on 8 May 1916.

Mallin was executed by firing squad on 8 May 1916. The presiding officer at his court martial was Colonel EWSK Maconchy. The night before his execution he was visited in his cell by his mother, three of his siblings, his pregnant wife and their four children. In his last letter to his wife, who was pregnant with their fifth child, Mallin stated that “I find no fault with the soldiers or the police” and asked her “to pray for all the souls who fell in this fight, Irish and English.” He commented “so must Irishmen pay for trying to make Ireland a free nation.” He wrote to his children ‘Una my little one be a Nun, Joseph my little man be a Priest if you can, James & John to you the care of your mother make yourselves good strong men for her sake and remember Ireland’ Both Una and Joseph followed his wishes. His funeral mass took place at the Dominican Church in Tallaght on 13 May 1916. People from the procession clashed with police outside the church with two policemen injured.