Reid, who was poached by Davis in 2001 to help turn the flailing business around, has decided not to take up his post on the rejigged board.
He will instead stay on as a consultant to the new merged business, Glencore-Xstrata, for a maximum of six months before exiting.
Davis said: “Eleven years ago, I persuaded Trevor to leave a successful banking career to embark upon the transformation of a struggling company in a precarious financial position with limited options.
“Xstrata’s evolution into one of the world's leading mining companies has exceeded even our initial ambitions.”
Reid’s decision comes on the back of Mick Davis’ exit deal agreed in September under a deal designed to placate shareholders.
Since then, Xstrata chairman Sir John Bond has also resigned. Glencore chief executive Ivan Glasenberg is to be installed as the leader of the merged business. A new finance chief is yet to be announced.
The exodus of Xstrata’s key personnel from the new business will sound alarm bells to investors who have previously questioned Glencore’s capacity and expertise to run the hands-on mining side of the business.