Gilmore, who is credited with being the agency’s creative engine during his 15-year stint, said he would still freelance for the agency but his role is to be scrapped.
His departure comes a month after managing director Rosie Doggett left.
Doggett was enlisted as its first managing director from Saatchi & Saatchi a year ago to lead an ambitious expansion plan.
That included taking on ten new staff and moving into new areas such as advertising, marketing services and even property.
Borkowski account director Joanne McNally said: "Mike has left by mutual consent. He was a writer here but that is now spread out across the business."
Gilmore said: "I have got various freelance jobs including a book with Mark [Borkowski]."
"I have simply reconfigured my working arrangements," he added.
The 26-strong agency is refocusing its business to concentrate on its main consumer clients including Vodafone.
Up to 20% of its work is focused on arts and entertainment clients such as circus act Cirque du Soleil.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.