She will be replaced by current managing director Tristia Harrison, the broadband provider said, reports the BBC.
TalkTalk also announced that founder Sir Charles Dunstone will become executive chairman after stepping down as chairman of Dixons Carphone in May.
TalkTalk, recently fined over a major hacking breach, also said that revenues fell in the final months of 2016.
Ms Harding, a member of the House of Lords and a trustee of digital inclusion charity Doteveryone, said she wanted to focus more on her public service activities.
“Between now and May, we will work together on a handover that maintains focus on this year’s performance and enables the new team to prepare for the next financial year and beyond,” she added.
Ms Harding, appointed in October 2010, was chief executive in 2015 when hackers attacked the company’s website, stealing confidential customer data.
TalkTalk estimated the cyber-attack, which divulged nearly 157,000 users’ financial details, cost it £42m.
The company was fined a record £400,000 in October for poor website security which led to the theft, making it the largest ever fine imposed by The Information Commissioner’s Office.
Sir Charles, who created TalkTalk in 2002, said Ms Harding had “helped transform TalkTalk into a much stronger business”.
“I’m excited at the prospect of spending more time on TalkTalk,” he added.
Sir Charles, the founder of Carphone Warehouse, will remain as a senior adviser to Dixons Carphone.
He said the firm, which was created via a merger between Dixons and Carphone Warehouse three years ago, remained “a very important part of my life”.
“I remain a committed shareholder and, in my new role as a senior advisor, I will continue to take an active interest in the business,” he added.
Sir Charles will be replaced as chairman of Dixons Carphone by Lord Livingston, a former chief executive of BT.
Meanwhile, the company announced that revenues had fallen to £435m, from £459m for the same period a year ago.
TalkTalk blamed the fall on a high level of contract renewals and new price deals, but said the impact of this was “short-term”.
