Collins was born in Woodfield, County Cork, the youngest of eight children. He moved to London in 1906, to become a clerk in the Post Office Savings Bank at Blythe House. He was a member of the London GAA, through which he became associated with the Irish Republican Brotherhood and the Gaelic League. He returned to Ireland in 1916 and fought in the Easter Rising. He was subsequently imprisoned in the Frongoch internment camp as a prisoner of war, but was released in December 1916.