The WPP-owned agency saw off competition from WWAV Rapp Collins in a final shoot-out. The client had also seen presentations from Rapier and the incumbent, Tequila Manchester.
HTW will now set to work on a global affinity marketing strategy designed to boost sales of the luxury £250,000 Phantom marque.
It also expected that the agency will be responsible for the worldwide launch of a new Rolls-Royce line that is expected to make its debut at motor shows next year.
Last year, Rolls-Royce failed to hit its self-imposed sales target of 1,000 Phantoms per year, managing to sell only 792 cars.