He was elected as a Sinn Féin Member of Parliament (MP) for the constituency of Sligo South at the 1918 general election. In January 1919, Sinn Féin MPs refused to recognise the Parliament of the United Kingdom and instead assembled at the Mansion House in Dublin as a revolutionary parliament called Dáil Éireann, though McCabe did not attend as he was in prison.
At the 1921 Irish elections, he was re-elected for Sligo–Mayo East. He supported the Anglo-Irish Treaty and voted in favour of it. He was again re-elected for Sligo–Mayo East at the 1922 general election, this time as pro-Treaty Sinn Féin Teachta Dála (TD). At the 1923 general election, he was elected as a Cumann na nGaedheal TD for Leitrim–Sligo. He resigned from Cumann na nGaedheal in 1924 because of dissatisfaction with government attitude to certain army officers and joined the National Group led by Joseph McGrath.
He resigned his Dáil seat in March 1925 along with several other TDs, and at the resulting by-election on 11 March 1925 was won by the Cumann na nGaedheal candidate Martin Roddy. He did not stand for public office again and returned to his post as a schoolteacher. He was involved with the short-lived but widely followed Irish Christian Front, serving as the organisation's secretary.
- Todd Andrews (1979), Dublin Made Me.
- Robert Fisk (1983), In Time of War.
- "Mr. Alexander McCabe". Oireachtas Members Database. Retrieved 11 April 2009.
- "Roll call of the first sitting of the First Dáil". Dáil Éireann Historical Debates (in Irish). 21 January 1919. Retrieved 11 April 2009.
- "Alexander McCabe". ElectionsIreland.org. Retrieved 11 April 2009.