Austin Stack was born in Ballymullen, Tralee, County Kerry. He was educated at the Christian Brothers School in Tralee. At the age of fourteen he left school and became a clerk in a solicitor's office. A gifted Gaelic footballer, he captained the Kerry team to All-Ireland glory in 1904. He also served as President of the Kerry Gaelic Athletic Association County Board.
He became politically active in 1908 when he joined the Irish Republican Brotherhood. In 1916, as commandant of the Kerry Brigade of the Irish Volunteers, he made preparations for the landing of arms by Roger Casement. Although he was made aware that Casement was arrested and was being held in Tralee, he made no attempt to rescue Casement from Ballymullen Barracks at this time. District Inspector Kearney (RIC) treated Casement very well and made sure Stack was aware that Casement could so easily have been rescued, yet Stack refused to move. He (Stack) was arrested and sentenced to death for his involvement, however, this was later commuted to penal servitude for life. Stack was released under general amnesty in June 1917 and was elected as an abstenionist Sinn Féin Member of Parliament for Kerry West in the 1918 Westminster election, becoming a member of the 1st Dáil. He was automatically elected as an abstenionist member of the House of Commons of Southern Ireland and a member of the 2nd Dáil as a Sinn Féin Teachta Dála for Kerry–Limerick West in the Irish elections, 1921.
He was elected to the 3rd Dáil at the 1922 general election and subsequent elections as a Anti-Treaty Sinn Féin TD for the Kerry constituency. When Éamon de Valera founded Fianna Fáil in 1926, Stack remained with Sinn Féin being re-elected to the Dáil in the June 1927 general election. He did not contest the September 1927 general election