The agency picked up the business following a four-way shoot-out against Chemistry and the incumbents, WWAV Rapp Collins and Craik Jones Watson Mitchell Voelkel.
Orange's decision to consolidate into HTW will come as a blow to Craik Jones, which has held Orange Mobile's DM account since 2001.
The WPP-owned HTW will now take on responsibility for Orange Broadband's below-the-line marketing, which has been handled by WWAV since it rebranded from Wanadoo in May 2006.
The win marks a change in fortunes for HTW, which has recently suffered the loss of several blue-chip clients as well as the departure of its co-founder and worldwide creative director, Steve Harrison.
Spencer McHugh, the head of direct and digital marketing at Orange, said: "Craik Jones and WWAV have done some wonderful work, but we felt it was time for a change."