The FTSE 100 electricity and gas network firm selected Brunswick following a three-month process that involved as many as 15 agencies.
It is thought that most of the City's key financial PR players pitched for the business, one of the few large-scale FTSE 100 reviews of 2011.
However, sources close to the process said that the pitch came down to incumbent Brunswick and recently launched StockWell Group, whose founders include former Brunswick partner Tim Burt.
Chris Mostyn, head of corporate media relations at the National Grid, confirmed that Brunswick had been retained to provide financial and strategic corporate comms support. He added that the grid's procurement rules required the company to launch the competitive tender process.
Brunswick has held the account since 2007, when it picked it up from Citigate Dewe Rogerson.
A key comms challenge for 2012 will be the upgrade of the UK's energy network to handle the electricity that the wind energy industry aims to produce, as well as convincing stakeholders of the need for new overhead cabling and pylons across the country.