Merrell joins Citigate as a director in its financial division after two years at Cubitt, starting on 7 October.
She was a director in Cubitt's private equity and capital markets division and has been replaced by Abigail Munson, who joins the agency in the same role on Monday.
Munson spent ten years at the BBC, mainly as a journalist, which included time as a senior outside broadcast producer on financial and general news for the BBC's major news programmes.
Merrell is also a former journalist, having worked for more than a decade at the The Times, covering banking and private equity, as well as stints at The Independent, the BBC and Money Marketing. She joined Cubitt from Temple Bar Advisory.
Citigate recruited Merrell to replace Sarah Gestetner, who joined private equity firm Warburg Pincus last week.
Gestetner joined the firm, a Citigate client, as its London-based comms director managing European press relations, strategic comms and marketing for the firm and its companies.
Before joining Citigate in 2001, Gestetner spent five years in corporate finance at Lazard.