Prior to joining Portland and before being appointed FCO high commissioner, Dean spent over two years as deputy director Europe for the office, following a year as its head of strategic finance.
She has also recently worked with the newly created Department for Exiting the European Union, and has served at embassies in Paris, Brussels and Washington.
The division Dean leads has been created to help clients navigate through the UK's current political landscape.
It includes old and new figures from the agency such as Stephen Wall, Portland's chief EU advisor and the UK's former representative to the EU; Portland managing partner and former Number 10 EU advisor Steve Morris; former treasury official Matt Pollard; and former Polish foreign minister Radek Sikorski. The latter two both joined Portland in the summer, when the Brexit unit was launched.
The unit also includes James Starkie, who was head of regional press for the Vote Leave campaign, and Amy Richards, former national press officer manager for campaign group Britain Stronger in Europe.
Dean said: "There are immediate issues, issues for transition, issues for the long term. In a sense, Brexit will never be over. Countries which are outside the EU still need to pay close attention to EU regulations and decisions which affect their business, so this is an area where our clients will need advice for many years to come."
During October, Omnicom announced it would be taking full ownership of the Portland, buying the remaining minority stake that it did not already own. Omnicom initialy acquired its stake in Portland in 2012.
Portland has more than 200 staff and is the world's 52nd largest and the UK's 16th largest agency by revenue, according to PRWeek's Global Agency Business Report.