Daniel Mannix was one of the most prominent opponents to conscription in World War One. He announced his position late in the first referendum debate, and was particularly prominent in the second referendum.

Daniel Mannix was born in County Cork, Ireland, in 1864. He studied at Maynooth Seminary where he was ordained a priest in 1890, and later occupied the chair of moral theology, and the Presidency from 1903. In 1913 Daniel Mannix was appointed as Co-Adjutor (i.e. assistant and successor) Archbishop in Melbourne, to help Archbishop Thomas Carr – then 74 years old - in his duties. Carr died in May 1917 and Mannix succeeded him as Archbishop. One of the reasons that Mannix was considered unsuitable for appointment to an Irish see was that he was considered too moderate on the issue of home rule by many Irish nationalists.

Mannix initially supported the war effort, but gradually grew more strident in his opposition to the war. The Easter Rising of 1916 clearly shaped his attitude. While describing the rebellion as “deplorable” Mannix also laid most of the blame for the situation on the British government and its Irish supporters.

Despite his prominent role, it is arguable as to how influential Mannix's intervention was in the final result. Maurice Blackburn thought that Mannix may have pushed as many Protestants into voting 'Yes' as he convinced Catholics to vote 'No'. Others have suggested that predominantly working-class Catholics were inclined to vote 'No' even before Mannix's speeches. On the other hand, Mannix almost certainly gave Australian Catholics more confidence in these views. 

In later life, Daniel Mannix was associated with anti-communist movements within the ALP, and the breakaway Democratic Labor Party.