Harding will join TalkTalk next spring, and will oversee the broadband business, which is soon to be the demerged broadband arm of Carphone Warehouse.
Carphone Warehouse’s chairman John Gildersleeve, who joined the business from Tesco, previously looked after the role.
TalkTalk will be Britain’s biggest residential broadband provider when it demerges from the retail arm of Carphone Warehouse early next year. Harding will leave Sainsbury’s next spring.
TalkTalk recently unveiled a scheme, which will see homeowners receive £250 for incorporating the TalkTalk brand into the formal address of their dwelling for one year.
The broadband provider has more than 4.1 million customers and is the second-largest broadband provider in the UK after BT.
It was the official sponsor of ITV’s reality programme, The X Factor, a deal which it took over from parent company The Carphone Warehouse.
The 2009 show has been the most watched ever series of The X Factor and ITV1 has registered average viewing figures up 2m on last year to 11.8m, equivalent to a 45.4% share.