The 64-year-old, who has served the island for 11 years, said he had thoroughly enjoyed his 'full and hectic' time in such a distinctive and unique place to live.
Mr Mellor said his replacement would be appointed by the Crown, but no decision was likely until after he had left.
Guernsey's vice-deans would take over responsibility in the interim, he said.
'It has been interesting and challenging for me here. There have been a lot of interesting people and situations. I think it is a very distinctive place and, from the church's perspective, it is quite a unique place.
'It is also a very beautiful place to live. Having come from Cornwall, it shares quite a lot in common in many ways, but I don't think people in the mainland are fully aware of how different Guernsey is.'
