Liberty, which bought Virgin Media in Britain as part of a European expansion last year, said it did not intend to make an offer for the rest of ITV, but shares in the group still rose 9 percent at the open as investors digested the move.
The sale, at a price of 481 million pounds ($824 million), may also indicate that BSkyB is looking to raise cash to push ahead with its own expansion plans after it opened talks this year to buy Rupert Murdoch's Sky Italia and Sky Deutschland.
The move for the ITV stake is just the latest in a string of deals in the global media sector motivated by a desire to own content. Liberty recently teamed up with Discovery Communications to buy TV producer All3Media and Murdoch's 21st Century Fox is seeking to buy Time Warner.
"It is hard to see how this will not be looked at as anything other than Liberty planting a flag in the ground for a potential acquisition at some point," Liberum analyst Ian Whittaker said.
"Liberty's Virgin Media did look to buy ITV nearly a decade ago and presumably the rationale for a deal then - access to content, help to support the Virgin brand and grow customers, financial engineering - still exists now."
ITV, known for broadcasting Britain's Got Talent and Downton Abbey, has been rebuilt under Chief Executive Adam Crozier to sell more programming around the world to make it less dependent on volatile advertising revenues.
With shares up 6 percent at 195 pence at 0736 GMT, ITV had a market valuation of 7.8 billion pounds.
The stake sale brings an end to one of the most controversial corporate moves in the British media sector - the decision by the then BSkyB boss James Murdoch to buy 17.9 percent of the then struggling company to block rival NTL from creating a larger, more powerful competitor.
The move, in 2006, kicked off a long-running regulatory investigation which eventually forced Murdoch to reduce his stake to 7.5 percent.
BSkyB bought the shares in 2006 for 135 pence, sold a more than 10 percent stake at a deep loss priced at 48.5 pence and sold the remainder this week at 185 pence.
NTL went on to become Virgin Media, which was then bought by Liberty. Liberty Global, controlled by billionaire cable executive Malone, has been expanding in Europe where it makes the bulk of its revenues.
"This is an opportunistic and attractive investment for us in our largest cable market," said Mike Fries, Liberty Global's chief executive. "ITV is the leading commercial broadcaster in the UK."