The two firms will retain their separate trading names in the UK and US as part of the deal.
Although Morgan Allen Moore will become part of the Dewey Square Group, MD Steve Morgan, who takes a seat on the group’s international board, stressed that the deal was a merger, not a takeover.
Morgan added that the move followed on from a five-year referral relationship with the US lobbying firm.
Morgan Allen Moore clients include Johnson & Johnson, Tesco and Thames Water. It has around 23 staff in offices in Cardiff, London and Tunbridge Wells.
The Dewey Square Group has around 40 staff in offices in Boston, Washington DC, Tampa and Sacramento.
Morgan has been associated with the Labour party since the 1997 general election, when he helped organise events outside London. He also campaigned for Al Gore, during his unsuccessful presidential candidacy.