Sandberg founded College Hill in 1990 and sold a majority stake to private equity firm Vitruvian Partners last October.
He will continue to provide support to a select group of the agency's clients and work in a senior consultancy role.
Sandberg said: 'I'm proud to have played my part in building the company that I founded into the group that it is today. It's been a fantastic journey. I believe the building blocks are in place for an exceptional future.'
He told PRWeek he was 'considering a number of options', most likely to be mainstream business strategy roles. 'I'm certainly not retiring,' he added.
As part of the Vitruvian deal, which valued the group at around £45m, Sandberg handed day-to-day management of the group to CEO Richard Nichols.
Managing partners Mark Garraway and Matthew Smallwood will continue to manage College Group's financial PR business.