Born in Kilkenny in 1893, Fitzpatrick lived in Dublin and was described as "a man with the common touch and a good organizer.
Fitzpatrick was a full-time official of the Grocers' trade union and secretary of its social club at the Banba Hall in Dublin's Parnell Square.
He was also involved in Comhairle na Poblachta, a body set up the same year to heal the rift between the military and political anti-Treaty forces in Ireland.
In 1931 Fitzpatrick was elected to the National Executive board of Saor Éire - a far-left political organisation established in September 1931 by communist-leaning members of the IRA.
Fitzpatrick succeeded Tom Barry as Chief of Staff in 1937, only to be ousted by Seán Russell at the 1938 GAC.