Dennis, who as editor at the energy giant has edited its annual report, joins as the agency looks to grow its corporate credentials.
His appointment follows a number of joint consumer and corporate wins for 3 Monkeys in recent months, including United Biscuits and The Technology Strategy Board.
Dennis is a former city correspondent for The Sunday Telegraph, who since going into PR in 2006 has been head of media at Freud Communications and head of news at the CBI.
3 Monkeys MD Christine Jewell said: "Guy brings a wealth of experience not only in terms of corporate PR but as a national newspaper journalist who has worked across key business sectors.
"We are currently enjoying very healthy momentum on the corporate business side and it’s great to have Guy on board to help keep us moving in the right direction."
Last month, the agency announced it had hired Howard Bowden as head of consumer content.