The purchase price was not disclosed.
The Mercator acquisition, which is subject to regulatory approval, is the First Names Group's fourth since its management buyout in July 2012, and major rebrand in early 2013. When it has completed, the company will employ more than 520 employees in all its offices.
Today's announcement of the acquisition comes less than six months after First Names revealed it had bought Basel Group, a Jersey-based trust provider, which it said at the time would ultimately result in the company becoming one of the world's largest operators in the offshore corporate services sector.
The other acquisitions, both last year, were of boutique fund services business Moore Management and international trust and corporate services provider Citadel.
First Names said it will incorporate Mercator's trust and corporate services business into its own trust and corporate services operations, while Mercator’s funds business will be incorporated into the Moore brand.
Mercator dates back more than 30 years, with its roots in a fiduciary business that had been part of Chandlers Chartered Accountants expanded and developed independently. It branded as Mercator in the 1990s.
In Guernsey, David Preston will continue as the managing director of Mercator's trust and corporate business, while company director Mark Douglas will remain in charge of the fund services business, operating under the Moore brand.
In other changes at First Names, Mark Pesco, the current managing director of Jersey, is to become managing director of the company's pan-Channel Islands trust and corporate businesses.
Client services director Ben Newman is being promoted to deputy managing director of First Names (Jersey), while and Peter Clarke, director of compliance in Jersey, will become head of compliance for the all of the Channel Islands.
Similarly, Mercator's chief operating officer, Sharon McMillan, is being named operations director for the Channel Islands.