The Llandaff Diocesan service begins at 15:00 GMT on Sunday at Llandaff Cathedral, ahead of Dr Morgan's retirement on Tuesday.
He has been at the head of the Church in Wales for nearly 14 years - the longest serving in the Anglican Communion.
He is retiring on his 70th birthday.
Dr Morgan has supported numerous causes, including same-sex marriage and more powers for the assembly.
The service will begin with a procession of 120 robed clergy, and Paul Marshall of the Church in Wales will pay tribute to Dr Morgan and present him with a gift on behalf of the diocese.
A farewell service is being held in Cardiff to mark the retirement of the Archbishop of Wales and Bishop of Llandaff, Dr Barry Morgan.