[5][6] His family moved to 10 Annadale Avenue in the Fairview area of Dublin in February 1895, when his father was transferred there; Mellows remained in Wexford with his grandfather, Patrick Jordan, due to ill health.

He attended the military school in Wellington Barracks in Cork and the Portobello garrison school in Dublin, but ultimately refused a military career much to his father's disappointment, instead working as a clerk in several Dublin firms, including the Junior Army & Navy Stores on D'Olier Street.

In 1911, he purchased a copy of Irish Freedom, which began: "We stand for Ireland..." He approached Thomas Clarke in his shop, who recruited him into Fianna Éireann,[7] an organisation of young republicans.

[9] In June 1913, Mellows stayed at Waterford IRB Centre with Liam Walsh in the Gaelic League Room, Williams Street.

He was active in the IRB and was a founder member of the Irish Volunteers, being brought onto its organising committee to strengthen the Fianna representation.

Mellows joined forces with Éamonn Ceannt and Sir Horace Plunkett at the Provisional Committee choosing to support the parliamentary route taken by Roger Casement.

On 1 August, Mellows' company was taking an arms shipment from Howth to Dublin and fired on the RIC at Clontarf.

He led roughly 700 Volunteers in abortive attacks on Royal Irish Constabulary stations at Oranmore, and Clarinbridge in county Galway taking over the town of Athenry.

However, his men were very badly armed and supplied and they dispersed after a week, when British troops and the cruiser Gloucester were sent west to attack them.

Mellows and a number of other local IRA leaders were able to evade capture after the station master of Athenry, James Miggins, delayed a detachment of incoming British troops.

[10] After this insurrection failed, Mellows escaped to the United States, where he was arrested and detained without trial in the "Tombs" prison, New York, on a charge of attempting to aid the German side in the First World War.

This was in the context of incidents like the Black Tom and Kingsland explosions, where German agents had bombed neutral American ports and industrial facilities.

After his release in 1918, he worked with John Devoy and helped to organise Éamon de Valera's fund raising visit to America in 1919–1920.

[13] A conference of nine TDs was deputed to meet privately on 5 January 1922 to resolve the dispute and to achieve a unified front by compromise.

[16]Mellows, the quartermaster-general with the rank of commandant general,[17] was one of a number of senior figures in the Anti-Treaty IRA faction that had occupied the Four Courts since April.

These executions, and their effects on their fellow prisoners, are described in Peadar O'Donnell's Irish Civil War memoir The Gates Flew Open.

In this document, Mellows outlines a ten-point programme he proposed that the Anti-Treaty IRA adopt in 1922:[19] Mellows wrote that "James Connolly realised that if Ireland were really to be free, it must be owned by the Irish people; that it was little use freeing Ireland from foreign tyranny if, in the course of a comparatively short time, it would fall under domestic tyranny as other countries had done".

[28] A socialist faction of the IRA in the late 1920s and 30s used the writing of Connolly and Mellows as their ideological basis and launched the political party Saor Éire.

[29] Writing for An Phoblacht in January 2010, Sinn Fein's Eoin Ó Broin also discussed Mellows' Marxist political positions.