The French-owned energy giant, which supplies electricity and gas to around five million British homes and businesses, previously worked with MHP and predecessor firms on public affairs, and with Brunswick on its corporate media relations.
Both incumbents had been working with the company for at least a decade, and both were in the running for the new consolidated brief, as PRWeek reported in March.
Portland has now claimed the account, ahead of a strong bid by Edelman, led by new public affairs chief Will Walden.
Alex Bigg, CEO of MHP Communications, said: "MHP is very proud of our work over a number of years to support EDF Energy, particularly in helping them make the case for new nuclear energy in the UK. We wish them well for the next steps on this journey."
MHP, which brought in former energy secretary Ed Davey as an adviser in early 2016, is understood to have recently won another significant piece of energy work, details of which are due to be made public shortly.
Brunswick declined to comment.
The win increases the momentum of Portland's progress to becoming a UK top 10 agency. Click for the profile on the firm from the 2017 PRWeek Global Agency Business Report.