Dana Gloger, who joined The Mail on Sunday in June, will join the Omnicom-owned agency in January.
She was previously consumer affairs editor at the Daily Express and has nine years' consumer reporting experience.
Gloger will sit in Portland's corporate team, acting as a media consultant to clients and reporting to Adrian Warr, partner for comms.
Tim Allan, Portland's founder, said that hiring Gloger was a 'statement of intent for 2013 that we'll continue to invest in the best talent to help us grow'.
The agency said Gloger would bring a 'new set of skills and a different point of view' to the team, having focused as a journalist on consumer products, trends and health issues, as well as business and personal finance.
Portland has continued to grow its corporate business in conjunction with its political work, winning Barclays' Heathrow, Funding Circle and Nestlé in 2012.
It was named as mid-sized agency of the year at the 2012 PRWeek awards in October.