This means Grey will handle creative work for all its brands including Twiglets, Mini Cheddars and Oddities. The appointment comes ahead of a new masterbrand campaign for Jacob's that will launch in 2016.
It won the account after it beat fellow WPP agency JWT in the final stage of a pitch process run by Roth Observatory.
The incumbent, Publicis London, did not repitch for the business.
Grey London already works on the McVities brand, which is also owned by United Biscuits.
Jeff van der Eems replaced Martin Glenn as the chief executive of United Biscuits in April. United Biscuits was acquired by Yildiz Holding, a Turkish company, in 2014.
Grey London and United Biscuits could not be reached for comment at the time of writing.
"We saw some fantastic ideas from all of the agencies in the pitch process, but Grey London’s work stood out from the moment we saw it," said Ted Linehan, the director of savoury brands at United Biscuits.
"Within United Biscuits, we've seen how great campaigns can connect consumers to our brands at an emotional level. The new campaign reflects our ambition to continue growing our Savoury business and modernise the Jacob's brand," he added.
Lucy Jameson, the chief executive at Grey London, said: "We are delighted that we've won this pitch. Not only do we get to work on yet another portfolio of great, iconic brands but we're also consolidating and growing our relationship with United Biscuits – one of our most exciting and creatively ambitious clients – as they invest and grow their business."