The organisation is looking for a successor to Halmos, who is due to retire on 31 October, under the modified job title of director of comms.
Halmos joined the City of London Corporation in 1994 after holding various organisational and campaigning roles at the Social Democratic Party, and before that at the TUC.
He told PRWeek: "I’m sad to give up an exciting and fascinating job after so long but also looking forward to new activities in the context of retiring from full-time employment."
Halmos said he hoped to have roles in think-tanks or academic positions after he retires.
The City of London Corporation is the municipal authority for the Square Mile. Its role is also to support and promote the City as a global financial and business centre.