Arnold has been brought on board to head the agency's specialist media relations offering.
He is charged with handling traditional print and broadcast media and will also be responsible for driving the firm's internet comms, including social media services.
Arnold will also handle Chelgate's client media training services.
He joins the agency from his role as MD of the Arnold Group. He will report to Chelgate chairman Terence Fane-Saunders.
Arnold was previously described by ABC News of America as 'one of Europe's leading spin doctors'.
His City experience involves advising clients including the Association of Chartered Certified Accountants, Amicus the Finance Union and the Financial Services Commission of Gibraltar.
He has also previously worked in the crisis comms arena on clients including water companies, railway firms, multi-national conglomerates and the Port of London Authority.
As a journalist Arnold worked as a BBC television reporter in Belfast and also contributed to ABC New York. He was also the first solo news anchor on Sky News.