The brothers want to take advantage of moves by companies such as Procter & Gamble and Unilever to cull their product portfolios to concentrate on superbrands.
The new venture, to be called Saatchinvest, begins life with two acquisitions from Heinz-Complan, the meal replacement drink, and Casilan, a protein supplement.
Bill Muirhead, one of the partners involved, insisted their roles were limited to private investment and that none had plans to leave the advertising business.
"There has been a lot of publicity about 'orphan' brands and about the venture capitalists who pick them up and flog them on. We aren't looking to do that, he said.
Muirhead said the agency would not automatically handle advertising for brands acquired by Saatchinvest because of possible conflict. If it did, the agency would charge the market rate.
The business will be run by Andrew Leeka, a former financier at Alchemy Partners, the UK private equity group. Its non-executive chairman will be Ken Manley, the former boss of the grocery products company Campbell.
Under the deal with Heinz, Saatchinvest will have 51 per cent of a joint venture which will acquire Complan and Casilan in Europe and the Americas.
Complan's management will hold a 15 per cent stake while Heinz retains 34 per cent.
Maurice Saatchi said: "We won't be looking for brands that don't have a decent share of their market."