He was a silk weaver, the co-founder with Francis Sheehy-Skeffington of the Socialist Party of Ireland, and was second-in-command of the Irish Citizen Army under James Connolly in the Easter Rising, in which he commanded the garrison at St. Stephen's Green in Dublin.

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'.

[citation needed] On Mallin's return to Ireland, he became a silk weaver's apprentice under his uncle James, who was also a former soldier in the British Army.

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.

[7] When Connolly was inducted into the Irish Republican Brotherhood (IRB) in January 1916, Mallin began preparing ICA members for the imminent armed revolution.

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.

[8] Mallin planned to occupy the Shelbourne Hotel, located on the north-east side of the park, but insufficient manpower prevented him from doing so.

This would prove disastrous for the revolutionaries as the British Army during the subsequent fighting was able to occupy the upper floors of the hotel on Monday night.

Early Tuesday morning the British Army forces in the Shelbourne began firing down on the encamped rebels.

Under intense fire, Mallin ordered his troops to retreat to the Royal College of Surgeons on the west side of the park.

Margaret Skinnider records in her autobiography that "I had ridden ahead to report to Commandant Mallin, and while he stood listening to me, a bullet whizzed through his hat.

The order to surrender was signed by James Connolly and P.H Pearse, delivered to Mallin by Nurse Elizabeth O'Farrell.

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.

[16] This court-martial record in itself was a double edged sword designed to discredit Mallin and at the same time indict Countess Markievicz by providing evidence that would confirm her execution.

General Maxwell, newly appointed Commander-in-Chief of British Army in Ireland, had already expressed his own motives for wanting to execute Markievicz.

[17] Maxwell needed a weight of evidence against Markievicz if he was to convince Prime Minister Asquith to accept his decision to confirm her death sentence.

[3] 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.

Despite having no memory of the visit to his father when he was 3 years old, he fulfilled one of Michael Mallin's last requests as written in his pre-execution letter to his family by joining the priesthood.