The upmarket retailer confirmed last week that Halliwell would leave after three years for a new opportunity.
Three has confirmed she will join the brand on 4 September. Halliwell succeeds Tom Malleschitz, who moved into the newly-created role of chief digital officer in December.
Dave Dyson, chief executive at Three UK, said: "We are delighted to have Shadi join our team. Her experience in building customer-centric brands will be a vital asset to our business as we continue to disrupt the industry and make it better for British consumers."
Halliwell is a veteran of the mobile industry. Prior to joining Harvey Nichols in April 2014, she spent 23 years at O2 and its predecessor BT Cellnet in a series of roles, eventually becoming head of brand.
During her time there, she oversaw the relaunch of the former Millennium Dome as The O2. The London entertainment complex is now the most visited venue in the world and O2 renewed its sponsorship in February in a ten-year deal worth about £125m.
Her era at Harvey Nichols has seen the retailer create a series of acclaimed campaigns with agency Adam & Eve/DDB.
The team won the Grand Prix in the Film Lions at last year’s Cannes for "Shoplifters", which imposed cartoon faces on real CCTV footage of thieves being apprehended.
Halliwell said: "Three has always been a pioneer in the telecommunications industry, with its bold approach to marketing and customer engagement setting it apart from its peers.
"I’m thrilled to be joining a team with such a high calibre of talent and creativity, and look forward to taking the brand further in the next chapter of its evolution."