In 1911, he entered the Civil Service and was assigned to the Board of Customs and Excise and a year later transferred to the finance division of the Chief Secretary's office in Dublin Castle.
[citation needed] During the July 1921 Truce, he was introduced to Michael Collins and later became a financial advisor to the team negotiating the Anglo-Irish Treaty.
[citation needed] In April 1922, he became the Irish Free State's first Comptroller and Auditor General and in April of the following year, he was appointed Secretary of the Department of Finance, a post he held until he retired from the Civil Service in 1927.
[citation needed] In 1925, his lengthy note on the Free State's financial position helped conclude the Irish Boundary Commission negotiations.
[3] In 1938, Joseph Brennan was conferred with an Honorary LLD by the National University of Ireland.