Berlin becomes the network's non-executive chairman.
Cameron takes over as United bids to establish a clear identity for itself after a strifetorn past and a series of false starts in previous incarnations as Conquest and Red Cell.
Despite having slimmed down to a nucleus of agencies in the US, Latin America and Europe, United has yet to realise the hopes of Sir Martin Sorrell, WPP's chief executive, that it should become the group's creatively led micronetwork.
It currently has no UK presence, having closed its London operation a year ago.
Cameron insisted United was financially sound and that the local agencies were delivering good margins.
But he acknowledged that winning new business was vital to its future credibility.
In future, the network is expected to extend what it claimed was a strong digital offering in the US across the network, target brands, such as luxury goods, that are likely to be heavily promoted online, and become a "creative engine" for major WPP clients.