Gladdis, who will join Engine-owned MHP in May, will provide counsel on media strategy, crisis comms and effective campaigning across all companies within the parent group, a spokesman for the agency said.
He will report to MHP head of media Ian Kirby - formerly of the News of the World - and work alongside director Adam Batstone, a former assistant editor at BBC News.
MHP CEO Alex Bigg said Gladdis had "knowledge, experience and passion for journalism", all of which would be a major benefit to the agency's clients. Gladdis will also be joining MHP's executive team, the agency confirmed.
Gladdis has been executive news editor at the Daily Mail since 2013, where he is responsible for the news, politics and campaigning content of the paper.
Before joining the Daily Mail in 2011 as senior news reporter, he was northern news editor at News of the World for five years, his LinkedIn profile says.
Prior to this, and ahead of a six-year stint as deputy political editor at News of the World, Gladdis was political editor at the Liverpool Daily Post and Echo between 1996 and 2000, his LinkedIn profile also states.
• Click here to subscribe to PRWeek – you'll get essential breaking news, views & expert analysis; an exclusive UK Daily News email bulletin; industry-leading reports such as the Power Book, Global Agency Business Report, Best Campaigns & Best Places to Work; and a whole lot more.