Popiolek joins the agency as MD, effectively replacing ex-CEO Nick Boakes, who left at the end of last year.
Grandfield decided to re-instate the MD role, also previously held by Boakes - who is now believed to have left the PR industry.
Popiolek takes on the role almost a year after his departure from GA, where he had headed the London operation for six years, first as MD and then as CEO (PRWeek, 1 March 2002).
He was replaced at GA by former Sunday Telegraph City editor Neil Bennett.
Popiolek said Grandfield was a 'fleet-footed firm' and his priorities were to grow the firm in corporate and financial PR.
'It's a challenging market and we will continue to service our clients intensively,' he said.
Grandfield has also appointed Popiolek as an equity partner in the firm.
Agency chairman Charles Cook said they will work as 'a duo', building the agency's domestic client list and recently launched pan-European advisory network, which covers Frankfurt, Paris, Geneva and Brussels (PRWeek, 14 February).
'I think in the current climate there's a particular opportunity for independent firms to take market share from some of the larger businesses that are subsidiaries of international groups,' said Cook.
'We are more focused - and own ourselves - and can take longer term decisions, because we have only ourselves to answer to,' he added.
Popiolek brings agency and journalism experience to the 17-strong firm.
Prior to GA, he worked as an account director at Financial Dynamics, before which he worked as a City journalist.
He trained on Investors' Chronicle, then went on to work at The Daily Telegraph, where he edited the Questor share-tipping column.