Roca will start work at Westminster on 12 July. The head of government affairs role there has been vacant since earlier this year.
While many councils rely on the Local Government Association to handle their lobbying activities, a growing number of larger councils are appointing public affairs experts to deal with their specific issues in Westminster.
In May, PRWeek reported that council comms heads had a raft of public affairs events planned to woo the new administration. Westminster Council was planning a drinks party at Westminster Abbey to build ties with the new breed of civil servants, MPs and ministers.
Essex does not plan to replace Roca, and his role will be covered by assistant chief executive for policy, community planning and regeneration Richard Pulestone.
Last year, Roca's comms team launched an agency, Essex Communications, to sell public relations and public affairs services to public and private bodies.
In March, Roca worked as interim head of comms at Suffolk County Council, dividing his time between Suffolk and Essex.
The news follows Westminster's appointment of London Evening Standard City Hall reporter Katharine Barney as a media officer. Barney joined the council after five years at the Standard.