The appointment is the precursor to a TV and press campaign set to break during the summer with the aim of turning the drink into a potent rival to Bacardi Breezer.
The campaign will mark the first time Grey London has worked directly for Diageo, the brand's owner, which normally works with the network's New York office.
The London shop is understood to have had difficulty cracking the brief for the UK launch, precipitating a pitch involving Mother and Bartle Bogle Hegarty. However, Diageo opted to stay with Grey after seeing presentations.
The new advertising will try to give the brand a distinctive male positioning - markedly different from Bacardi Breezer, which has a predominately female appeal.
Morgan's Original Spiced is a spin-off from the Captain Morgan's brand that joined the Diageo stable after the company acquired Seagram two years ago. A full-strength spirit made from Caribbean rum and spices, Original Spiced has been available in the UK for several years.
The launch work is expected to underscore its masculine theme by contemporising the eponymous Captain Morgan, the pirate who has appeared on the brand's bottles.