Marckini will lead teams in London and Boston, who will staff the new Isobar global search team. They will roll out iProspect's paid search marketing tools including iProspect's search bidding agent iSEBA, through the Isobar network.
Marckini founded iProspect in 1996 and sold it to Isobar in 2004 for an estimated $50m (£26m). iPropsect president Rob Murray will now take on the role of chief executive.
Nigel Morris, chief executive of Isobar, said: "We are delighted that Frederick and Rob have elected to stay with Isobar and iProspect post-acquisition. Frederick will apply the same degree of innovation and vision to internationalising and unifying our global search offering that we have seen in the exceptional growth of iProspect in the US."
Marckini has written three search engine marketing books and has twice been on the list of BtoB Magazine's Top 100 marketers.