Catermasters has won a three-year catering contract with global marketing and communications firm Young & Rubicam.
The deal, with an option to extend for a further two years, is worth around £450,000 a year and sees Catermasters operate at the client's UK headquarters at Greater London House in Camden.
Young & Rubicam put the contract out to tender in late 2013 after 11 years with the previous supplier. Its facilities manager, Paul Whittle, said: "Catermasters ticked all of the boxes during the tender and have shown lots of hunger to succeed since coming on site.
"They are clearly passionate about great food and I think we're in good hands. We visited Catermasters' site at the British Heart Foundation as part of the tender process and we were very impressed with the set-up at what is almost a like-for-like site to ours."
The caterer has invested an undisclosed sum in building a new coffee bar, which Whittle said has added a sense of theatre to the staff restaurant.
Nigel Johnson, Catermasters joint managing director, said: "We aim to bring lots of enthusiasm, passion and skill to our sites because we know it ensures the best possible results. That will continue to be our approach at Greater London House."