His move to BMB will see him working alongside the agency's three founding partners, Trevor Beattie, Andrew McGuinness and Bil Bungay. As part of his package, Bain will receive an undisclosed equity stake in the start-up.
Bain joined Saatchis in May 2005 and was charged with leading the planning on the agency's Toyota Europe business, the agency's second-biggest-billing account, after Procter & Gamble.
Commenting on his move, Bain said: "BMB is the most exciting new agency in London. Opportunities like this genuinely happen only once in a career."
BMB has also made its first creative team signing, hiring Pat Burns and Gavin McGrath from TBWA\London.