Chris Liddle, who will remain chairman of HLM (UK), said that US expertise would be of benefit when bidding for healthcare projects. He said: "From now on we can offer a level of US healthcare consultation in-house, like some of our rivals."
Liddle, who said that the similarity of names was "amazing", added that the firms shared core strategies. He said: "We are always on the lookout for like-minded people, and we have found some like-minded people."
HLM (US) chief executive officer Joseph Harris said: "It's a wonderful coincidence for our branding strategy that we have the same name. Both firms are also recognised as market leaders serving healthcare, justice, education and corporate clients."
For HLM (US) the move is part of a global acquisition strategy. The firm also owns London-based practice GA Design.
It is thought that HLM (US) will use its UK arm's relationships with firms outside Europe to help set up further overseas acquisitions.
Harris and Vernon Brannon of HLM (US) and Werner Aeberhard of GA Design will join the UK board as directors. Liddle and Rod Fraser will join the board of the holding company, HLM International.
The two firms came across each other's websites in December 1999 and met a month later. For the past 18 months they have been pitching together for PFI projects.
HLM (UK) told staff about the acquisition last week. The company expects to increase its staff number beyond the 140 it currently employs. The firm has already employed five additional staff since January.
In October last year, the company cut its staff 10% following the completion of much of its PFI work.