Conor Pickering, who worked at GCI from 1998 to 2001, takes the role of country manager for the two Balkan states.
Reporting to CEO Alistair McLeish, Pickering last week took charge of seven staff in Bucharest and four in Sofia.
Pickering had spent the past two months in Sofia, leading MMD's crisis management work for US energy giant AES, which is investing more than £550m in the country. In Bulgaria, MMD also works for tyre giant Goodyear.
In Romania, clients include LNM Group, the steel conglomerate whose boss Lakshmi Mittal hit UK headlines in February.
It was revealed that Prime Minister Tony Blair had backed attempts by Mittal - who had donated £125,000 to the Labour Party - to buy state-owned steel firm Sidex in a letter to the Romanian prime minister.
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