[8] Harries was made deacon in 1963, becoming assistant curate of Hampstead St John in the Diocese of London (1963–69).

He was ordained priest the following year and later combined his ministry at St John's with the chaplaincy of the former Westfield College (now part of Queen Mary, University of London) (1967–69).

In the week prior to his retirement, on 26 May 2006, Downing Street announced that he was to be made a life peer, and he was gazetted as Baron Harries of Pentregarth, of Ceinewydd in the County of Dyfed on 30 June 2006.

He has been a member of the Human Fertilisation and Embryology Authority (including serving as chair of the HFEA Ethics and Law Committee) and a member of the Nuffield Council on Bioethics, as well as chairing the House of Lords select committee on stem cell research.

"[14] Harries currently serves as an Advisory Steering Group member for the Commission on Religion and Belief in British Public Life.

At the start of his episcopacy, he brought legal proceedings challenging the Church Commissioners' policy on investment.

Although this challenge failed – the Commissioners already had an ethical investment policy, albeit one which excluded a smaller part of the UK share market than the plaintiffs had wanted to exclude – the Court recognised that it was proper for charities to consider whether their investment strategies would alienate the charity's financial supporters.

[17] In 1996, Harries formed part of a working group of church leaders looking to address the increasing numbers of homeless in west London.

Harries and the other original founders championed the need for an open-access shelter that welcomed all in need, regardless of local connection, religion or nationality.

The group, formerly known as West London Churches Homeless Concern, gained charity status in 2000 and changed its name to "Glass Door" in 2014.

[18] In 2014, Harries stated in the House of Lords that the next British coronation in Westminster Abbey should feature readings from the Quran, the holy book of Islam.