Brennan's contact and involvement with Aboriginal Australians began early in his priestly ministry. In 1975 he worked in the inner Sydney parish of Redfern with priest activist Fr Ted Kennedy, where he also met and worked with Mum Smith among others who were founding indigenous Australian legal, health and political initiatives. In 1997, he was Rapporteur at the Australian Reconciliation Convention and the following year he was appointed an Ambassador for Reconciliation by the Council for Aboriginal Reconciliation. On 10 December 2008 he was appointed as the chairperson to the Australian Government's National Human Rights Consultation Committee. In 2009 this independent committee consulted with the Australian community about the protection and promotion of human rights. On 30 September 2009, it reported its recommendations to the Attorney General, the Honourable Robert McClelland MP. Brennan is a professor of law in the Public Policy Institute at the Australian Catholic University, a visiting professorial fellow at the University of New South Wales and served as the founding director of the Uniya Jesuit Social Justice Centre in Sydney from 2001 to 2007. In 2005, he returned to Australia from a fellowship at Boston College. During 2011, Brennan was critical of the refugee policies of the Prime Minister, Julia Gillard, saying that she has led the Labor Party into moral decline and that the Malaysia Solution is morally derelict and tantamount to "offshore dumping". On 15 August 2017 Brennan stated that if the law was changed to require clergy to report child sexual abuse learned of during confessionals he would consider breaking it. Brennan told ABC Radio National that "I as a Catholic priest would have to make a decision, whether in conscience, I could apply with such a law". He also claimed that "I think it would make children more vulnerable and not less". During the 2017 Australian Marriage Law Postal Survey, Brennan dissented from traditional Catholic teaching, telling the media he would vote yes and stating that "We've got to factor that in to the common good argument about what's necessary." He stated that, while in the context of Catholic marriage he would continue to uphold marriage as being between a man and a woman, he considered the issue of civil marriage to be separate. Following the survey, Brennan was appointed by Prime Minister Malcolm Turnbull to serve on a Philip Ruddock-led review into religious freedoms. In November 2019, it was announced that Brennan would be one of 20 members of the Senior Advisory Group to help co-design the Indigenous voice to government set up by Ken Wyatt, the Minister for Indigenous Australians. The Group is co-chaired by Wyatt, Marcia Langton and Tom Calma.
