He is due to start in April, replacing James Shelley, who has been in the role on a temporary basis from the Government Communications Service.
One of Portland’s longest serving senior partners, Orr has been at the agency since 2004, establishing the international team and leading many global campaigns.
International Trade Secretary Dr Liam Fox said: "Toby brings a wealth of experience in developing and implementing communication strategies in a global environment. As director of communications, he and his team will deliver a key corporate function for the Department for International Trade, communicating the department’s priorities to our target audiences at home and abroad."
Orr said: "It is an honour to be joining the department at such an important time, working to devise and deliver an ambitious communications strategy to advance the UK’s trade agenda and promote global Britain. I look forward to working closely with colleagues across the department and Whitehall on this exciting mandate.
"I have loved every minute at Portland, being part of a journey that has taken us from a handful of employees to a 200-strong company with offices across the world. No other communications agency gives you the chance to work with such a talented mix of people, doing such interesting work, for such prestigious clients."
Orr is the latest senior Portland figure to leave the agency for a role in government. Others have included Nick Hargrave, currently special adviser to Theresa May; and Jessica Seldon, senior press officer at Downing Street.
Senior Portland figures to have gone the other way in recent months have included James Helm, former director of comms at the Department for International Development and now a partner in its international team in London; and Victoria Dean, former British High Commissioner to Barbados & the Eastern Caribbean, who now heads Portland’s Brexit team.
Portland MD Tim Allan said: "I will be sorry to see Toby go, but I am incredibly proud of his Portland career. He started as an account exec in a slightly mouldy and soon-to-be-bulldozed office with five staff and a handful of clients. He has helped build a business which now has over 200 staff on five continents. He leaves behind a great team, an awesome client list and a legacy of innovative and creative work. It’s a credit to him that he leaves Portland for such a critical role at the heart of government."
Portland – whose management team includes former Downing Street comms figures Allan and managing partner Steve Morris – was recently named on the Government’s tactical agency roster.
PRWeek reported last year that Omnicom is to take full ownership of Portland, buying the remaining minority stake that it does not already own.