Greig was initially recruited by London & Partners last year to lead the media launch of Dot London, the new internet domain for the City, and was kept on to lead PR for its technology unit.
In his new role Greig will replace Tamzen Isacsson, who is leaving the company after two years.
Before joining London & Partners, Greig was an associate director with Newgate Communications. Prior to this he was a journalist in the BBC's business and economics division, serving as a senior producer with Radio 4’s Today programme for more than 11 years.
Chris Gottlieb, chief marketing and comms officer at London & Partners, said: "Martin's track record in both journalism and PR make him the ideal person to take over from Tamzen Isacsson, who has worked tirelessly over the past two years and delivered some outstanding results. We have consistently demonstrated, both during and since the Olympics, what can be achieved by working together with businesses and organisations across the capital. I look forward to us delivering more world-class campaigns and telling London's story brilliantly."
Greig said: "This is an exciting time to be part of London's story. From tech and life sciences innovators through to the capital's cultural institutions, London is alive with an energy and dynamism that is attracting record numbers of tourists, business and investment to the city. I'm delighted to have been given the opportunity to take the PR team forwards, building on our recent successes, creating innovative campaigns and communicating the positive messages about London to a global audience."
Over the past year London & Partners has led the launch of major initiatives including London Technology Week, MedCity, the London Story, the Autumn Season and London's Guest of Honour.