Councillor McCollum replaces David Rossiter – who informed Alliance he was stepping down from his role last month due to gaining new employment.
Following her selection Ms McCollum said: "Alliance provides strong representation across North Down and I’m excited to continue to work to deliver for everyone locally and representing the people of Holywood and Clandeboye, a place I proudly call my home."
A new face within the Alliance North Down team, John said he was honoured to represent the Bangor East and Donaghadee area, having spent most of his professional life in the area.
He added: “I’m absolutely delighted to be joining Councillor Hannah Irwin in delivering the vital work Alliance has been delivering in the area. I’m passionate about what we can do to help local people and am looking forward to getting out into the community to hear directly from local people.”