He was an Elder in the Presbyterian Church in Ireland until 2019 when he resigned citing the narrowing of spiritual outlook in PCI and its intolerance of minorities as the reasons for leaving.

He first ran for political office in Ballymena Borough Council elections while a medical student in 1989.

He welcomed many visitors and heads of state to the City in the period after the signing of the Good Friday Agreement, including Vaclav Havel, Mary Robinson and Bill Clinton.

[1][2] During his time as Lord Mayor of Belfast, dissident republicans detonated the Omagh bomb, the worst atrocity of The Troubles, killing 29 individuals.

As leader of the Alliance Party City Hall group, Alderdice took the controversial decision to vote for Councillor Alex Maskey of Sinn Féin to be Lord Mayor in 2002 which resulted in violence from loyalists upon his family and home.