PRWeek understands that the agency will be advising senior levels of the government, including the office of Prime Minister Abdullah bin Nasser bin Khalifa Al Thani.
The retained work was won through a pitch and is understood to cover a broad remit ranging from government affairs through to nation branding.
It comes as the country, which has the highest GDP per capita in the world, finds itself under increasing scrutiny internationally, and facing anger from its neighbours in the Middle East.
In March Saudi Arabia, Bahrain, the United Arab Emirates and Egypt withdrew their ambassadors from the country after accusations of Qatar backing Islamist groups including the Muslim Brotherhood, which has been violently suppressed by the new regime in Egypt since being overthrown by the military last year.
One agency head with experience of the Gulf told PRWeek: "The country has fallen out with its neighbours, and the government is clearly recognising there are major issues to tackle.
"It may be one of the richest, but the country has a lot to learn on the global stage, and in going for the work Portland will have had to have thought very carefully, as it could count against the agency in other markets in the region."
Qatar has also come under scrutiny over its successful World Cup bid following allegations of corruption against a Qatari former FIFA official recently published in The Sunday Times and concern over the rights of migrant workers hired to work on infrastructure for the competition in 2022.
It is thought that Portland's remit will not impact on the World Cup remit of fellow UK consultancy Blue Rubicon. The latter is understood to have been handed a retained brief to work with the Supreme Committee for Delivery & Legacy, the Qatari government’s World Cup organising committee, last September.
Portland has previously worked for the government of Dubai, and currently works for the government of Kazakhstan, the States of Jersey and the High Commission of Rwanda.
The agency was founded by Tim Allan, a former senior adviser to Tony Blair, and is now majority-owned by US marketing services company Omnicom.
The Qatari government had not yet responded to a request for comment, while both Portland and Blue Rubicon declined to comment.