Kleiner, who started this month, will advise clients on how to promote themselves, and how to enhance their reputations via a mix of social media and broader campaigns online.
He said: "It is increasingly important to integrate a well-positioned digital offering into all communications and marketing campaigns to enhance a firm's reputation via social media and wider online work. I look forward to further building Newgate and Publicasity's expertise in this area."
Kleiner will report to Newgate CEO and Porta chief operating officer Gavin Devine, who said: "Neil's arrival at Newgate represents another important step in the growth of our business. His expertise will be invaluable, both in terms of the advice he can give directly to clients and the way that he will improve the knowledge and capability of our whole team."
Prior to starting his new role, Kleiner was head of social and content at Golin, having spent two years previously as head of social media at the agency between 2014 and 2016. Before this between 2010 and 2014 he was head of social media at Havas Media UK.
Kleiner's appointment at Newgate follows that of ex-Bell Pottinger partner David Telling in January, and former Conservative special adviser Siân Jones, who joined the agency's public affairs practice in March.